Documentation/git-read-tree.txt - jrn/git - Git at Google

 git-read-tree(1)
 ================

 NAME
 ----
 git-read-tree - Reads tree information into the index


 SYNOPSIS
 --------
 [verse]
 'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
 		[-u [--exclude-per-directory=<gitignore>] | -i]]
 		[--index-output=<file>] [--no-sparse-checkout]
 		(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])


 DESCRIPTION
 -----------
 Reads the tree information given by <tree-ish> into the index,
 but does not actually *update* any of the files it "caches". (see:
 linkgit:git-checkout-index[1])

 Optionally, it can merge a tree into the index, perform a
 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
 flag.  When used with `-m`, the `-u` flag causes it to also update
 the files in the work tree with the result of the merge.

 Trivial merges are done by 'git read-tree' itself.  Only conflicting paths
 will be in unmerged state when 'git read-tree' returns.

 OPTIONS
 -------
 -m::
 	Perform a merge, not just a read.  The command will
 	refuse to run if your index file has unmerged entries,
 	indicating that you have not finished previous merge you
 	started.

 --reset::
         Same as -m, except that unmerged entries are discarded
         instead of failing.

 -u::
 	After a successful merge, update the files in the work
 	tree with the result of the merge.

 -i::
 	Usually a merge requires the index file as well as the
 	files in the working tree to be up to date with the
 	current head commit, in order not to lose local
 	changes.  This flag disables the check with the working
 	tree and is meant to be used when creating a merge of
 	trees that are not directly related to the current
 	working tree status into a temporary index file.

 -n::
 --dry-run::
 	Check if the command would error out, without updating the index
 	nor the files in the working tree for real.

 -v::
 	Show the progress of checking files out.

 --trivial::
 	Restrict three-way merge by 'git read-tree' to happen
 	only if there is no file-level merging required, instead
 	of resolving merge for trivial cases and leaving
 	conflicting files unresolved in the index.

 --aggressive::
 	Usually a three-way merge by 'git read-tree' resolves
 	the merge for really trivial cases and leaves other
 	cases unresolved in the index, so that porcelains can
 	implement different merge policies.  This flag makes the
 	command resolve a few more cases internally:
 +
 * when one side removes a path and the other side leaves the path
   unmodified.  The resolution is to remove that path.
 * when both sides remove a path.  The resolution is to remove that path.
 * when both sides add a path identically.  The resolution
   is to add that path.

 --prefix=<prefix>/::
 	Keep the current index contents, and read the contents
 	of the named tree-ish under the directory at `<prefix>`.
 	The command will refuse to overwrite entries that already
 	existed in the original index file. Note that the `<prefix>/`
 	value must end with a slash.

 --exclude-per-directory=<gitignore>::
 	When running the command with `-u` and `-m` options, the
 	merge result may need to overwrite paths that are not
 	tracked in the current branch.  The command usually
 	refuses to proceed with the merge to avoid losing such a
 	path.  However this safety valve sometimes gets in the
 	way.  For example, it often happens that the other
 	branch added a file that used to be a generated file in
 	your branch, and the safety valve triggers when you try
 	to switch to that branch after you ran `make` but before
 	running `make clean` to remove the generated file.  This
 	option tells the command to read per-directory exclude
 	file (usually '.gitignore') and allows such an untracked
 	but explicitly ignored file to be overwritten.

 --index-output=<file>::
 	Instead of writing the results out to `$GIT_INDEX_FILE`,
 	write the resulting index in the named file.  While the
 	command is operating, the original index file is locked
 	with the same mechanism as usual.  The file must allow
 	to be rename(2)ed into from a temporary file that is
 	created next to the usual index file; typically this
 	means it needs to be on the same filesystem as the index
 	file itself, and you need write permission to the
 	directories the index file and index output file are
 	located in.

 --no-sparse-checkout::
 	Disable sparse checkout support even if `core.sparseCheckout`
 	is true.

 --empty::
 	Instead of reading tree object(s) into the index, just empty
 	it.

 <tree-ish#>::
 	The id of the tree object(s) to be read/merged.


 Merging
 -------
 If `-m` is specified, 'git read-tree' can perform 3 kinds of
 merge, a single tree merge if only 1 tree is given, a
 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
 provided.


 Single Tree Merge
 ~~~~~~~~~~~~~~~~~
 If only 1 tree is specified, 'git read-tree' operates as if the user did not
 specify `-m`, except that if the original index has an entry for a
 given pathname, and the contents of the path match with the tree
 being read, the stat info from the index is used. (In other words, the
 index's stat()s take precedence over the merged tree's).

 That means that if you do a `git read-tree -m <newtree>` followed by a
 `git checkout-index -f -u -a`, the 'git checkout-index' only checks out
 the stuff that really changed.

 This is used to avoid unnecessary false hits when 'git diff-files' is
 run after 'git read-tree'.


 Two Tree Merge
 ~~~~~~~~~~~~~~

 Typically, this is invoked as `git read-tree -m $H $M`, where $H
 is the head commit of the current repository, and $M is the head
 of a foreign tree, which is simply ahead of $H (i.e. we are in a
 fast-forward situation).

 When two trees are specified, the user is telling 'git read-tree'
 the following:

      1. The current index and work tree is derived from $H, but
 	the user may have local changes in them since $H.

      2. The user wants to fast-forward to $M.

 In this case, the `git read-tree -m $H $M` command makes sure
 that no local change is lost as the result of this "merge".
 Here are the "carry forward" rules, where "I" denotes the index,
 "clean" means that index and work tree coincide, and "exists"/"nothing"
 refer to the presence of a path in the specified commit:

 	I                   H        M        Result
        -------------------------------------------------------
      0  nothing             nothing  nothing  (does not happen)
      1  nothing             nothing  exists   use M
      2  nothing             exists   nothing  remove path from index
      3  nothing             exists   exists,  use M if "initial checkout",
 				     H == M   keep index otherwise
 				     exists,  fail
 				     H != M

         clean I==H  I==M
        ------------------
      4  yes   N/A   N/A     nothing  nothing  keep index
      5  no    N/A   N/A     nothing  nothing  keep index

      6  yes   N/A   yes     nothing  exists   keep index
      7  no    N/A   yes     nothing  exists   keep index
      8  yes   N/A   no      nothing  exists   fail
      9  no    N/A   no      nothing  exists   fail

      10 yes   yes   N/A     exists   nothing  remove path from index
      11 no    yes   N/A     exists   nothing  fail
      12 yes   no    N/A     exists   nothing  fail
      13 no    no    N/A     exists   nothing  fail

 	clean (H==M)
        ------
      14 yes                 exists   exists   keep index
      15 no                  exists   exists   keep index

         clean I==H  I==M (H!=M)
        ------------------
      16 yes   no    no      exists   exists   fail
      17 no    no    no      exists   exists   fail
      18 yes   no    yes     exists   exists   keep index
      19 no    no    yes     exists   exists   keep index
      20 yes   yes   no      exists   exists   use M
      21 no    yes   no      exists   exists   fail

 In all "keep index" cases, the index entry stays as in the
 original index file.  If the entry is not up to date,
 'git read-tree' keeps the copy in the work tree intact when
 operating under the -u flag.

 When this form of 'git read-tree' returns successfully, you can
 see which of the "local changes" that you made were carried forward by running
 `git diff-index --cached $M`.  Note that this does not
 necessarily match what `git diff-index --cached $H` would have
 produced before such a two tree merge.  This is because of cases
 18 and 19 --- if you already had the changes in $M (e.g. maybe
 you picked it up via e-mail in a patch form), `git diff-index
 --cached $H` would have told you about the change before this
 merge, but it would not show in `git diff-index --cached $M`
 output after the two-tree merge.

 Case 3 is slightly tricky and needs explanation.  The result from this
 rule logically should be to remove the path if the user staged the removal
 of the path and then switching to a new branch.  That however will prevent
 the initial checkout from happening, so the rule is modified to use M (new
 tree) only when the content of the index is empty.  Otherwise the removal
 of the path is kept as long as $H and $M are the same.

 3-Way Merge
 ~~~~~~~~~~~
 Each "index" entry has two bits worth of "stage" state. stage 0 is the
 normal one, and is the only one you'd see in any kind of normal use.

 However, when you do 'git read-tree' with three trees, the "stage"
 starts out at 1.

 This means that you can do

 ----------------
 $ git read-tree -m <tree1> <tree2> <tree3>
 ----------------

 and you will end up with an index with all of the <tree1> entries in
 "stage1", all of the <tree2> entries in "stage2" and all of the
 <tree3> entries in "stage3".  When performing a merge of another
 branch into the current branch, we use the common ancestor tree
 as <tree1>, the current branch head as <tree2>, and the other
 branch head as <tree3>.

 Furthermore, 'git read-tree' has special-case logic that says: if you see
 a file that matches in all respects in the following states, it
 "collapses" back to "stage0":

    - stage 2 and 3 are the same; take one or the other (it makes no
      difference - the same work has been done on our branch in
      stage 2 and their branch in stage 3)

    - stage 1 and stage 2 are the same and stage 3 is different; take
      stage 3 (our branch in stage 2 did not do anything since the
      ancestor in stage 1 while their branch in stage 3 worked on
      it)

    - stage 1 and stage 3 are the same and stage 2 is different take
      stage 2 (we did something while they did nothing)

 The 'git write-tree' command refuses to write a nonsensical tree, and it
 will complain about unmerged entries if it sees a single entry that is not
 stage 0.

 OK, this all sounds like a collection of totally nonsensical rules,
 but it's actually exactly what you want in order to do a fast
 merge. The different stages represent the "result tree" (stage 0, aka
 "merged"), the original tree (stage 1, aka "orig"), and the two trees
 you are trying to merge (stage 2 and 3 respectively).

 The order of stages 1, 2 and 3 (hence the order of three
 <tree-ish> command line arguments) are significant when you
 start a 3-way merge with an index file that is already
 populated.  Here is an outline of how the algorithm works:

 - if a file exists in identical format in all three trees, it will
   automatically collapse to "merged" state by 'git read-tree'.

 - a file that has _any_ difference what-so-ever in the three trees
   will stay as separate entries in the index. It's up to "porcelain
   policy" to determine how to remove the non-0 stages, and insert a
   merged version.

 - the index file saves and restores with all this information, so you
   can merge things incrementally, but as long as it has entries in
   stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
   now the merge algorithm ends up being really simple:

   * you walk the index in order, and ignore all entries of stage 0,
     since they've already been done.

   * if you find a "stage1", but no matching "stage2" or "stage3", you
     know it's been removed from both trees (it only existed in the
     original tree), and you remove that entry.

   * if you find a matching "stage2" and "stage3" tree, you remove one
     of them, and turn the other into a "stage0" entry. Remove any
     matching "stage1" entry if it exists too.  .. all the normal
     trivial rules ..

 You would normally use 'git merge-index' with supplied
 'git merge-one-file' to do this last step.  The script updates
 the files in the working tree as it merges each path and at the
 end of a successful merge.

 When you start a 3-way merge with an index file that is already
 populated, it is assumed that it represents the state of the
 files in your work tree, and you can even have files with
 changes unrecorded in the index file.  It is further assumed
 that this state is "derived" from the stage 2 tree.  The 3-way
 merge refuses to run if it finds an entry in the original index
 file that does not match stage 2.

 This is done to prevent you from losing your work-in-progress
 changes, and mixing your random changes in an unrelated merge
 commit.  To illustrate, suppose you start from what has been
 committed last to your repository:

 ----------------
 $ JC=`git rev-parse --verify "HEAD^0"`
 $ git checkout-index -f -u -a $JC
 ----------------

 You do random edits, without running 'git update-index'.  And then
 you notice that the tip of your "upstream" tree has advanced
 since you pulled from him:

 ----------------
 $ git fetch git://.... linus
 $ LT=`git rev-parse FETCH_HEAD`
 ----------------

 Your work tree is still based on your HEAD ($JC), but you have
 some edits since.  Three-way merge makes sure that you have not
 added or modified index entries since $JC, and if you haven't,
 then does the right thing.  So with the following sequence:

 ----------------
 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
 $ git merge-index git-merge-one-file -a
 $ echo "Merge with Linus" | \
   git commit-tree `git write-tree` -p $JC -p $LT
 ----------------

 what you would commit is a pure merge between $JC and $LT without
 your work-in-progress changes, and your work tree would be
 updated to the result of the merge.

 However, if you have local changes in the working tree that
 would be overwritten by this merge, 'git read-tree' will refuse
 to run to prevent your changes from being lost.

 In other words, there is no need to worry about what exists only
 in the working tree.  When you have local changes in a part of
 the project that is not involved in the merge, your changes do
 not interfere with the merge, and are kept intact.  When they
 *do* interfere, the merge does not even start ('git read-tree'
 complains loudly and fails without modifying anything).  In such
 a case, you can simply continue doing what you were in the
 middle of doing, and when your working tree is ready (i.e. you
 have finished your work-in-progress), attempt the merge again.


 Sparse checkout
 ---------------

 "Sparse checkout" allows populating the working directory sparsely.
 It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
 Git whether a file in the working directory is worth looking at.

 'git read-tree' and other merge-based commands ('git merge', 'git
 checkout'...) can help maintaining the skip-worktree bitmap and working
 directory update. `$GIT_DIR/info/sparse-checkout` is used to
 define the skip-worktree reference bitmap. When 'git read-tree' needs
 to update the working directory, it resets the skip-worktree bit in the index
 based on this file, which uses the same syntax as .gitignore files.
 If an entry matches a pattern in this file, skip-worktree will not be
 set on that entry. Otherwise, skip-worktree will be set.

 Then it compares the new skip-worktree value with the previous one. If
 skip-worktree turns from set to unset, it will add the corresponding
 file back. If it turns from unset to set, that file will be removed.

 While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
 files are in, you can also specify what files are _not_ in, using
 negate patterns. For example, to remove the file `unwanted`:

 ----------------
 /*
 !unwanted
 ----------------

 Another tricky thing is fully repopulating the working directory when you
 no longer want sparse checkout. You cannot just disable "sparse
 checkout" because skip-worktree bits are still in the index and your working
 directory is still sparsely populated. You should re-populate the working
 directory with the `$GIT_DIR/info/sparse-checkout` file content as
 follows:

 ----------------
 /*
 ----------------

 Then you can disable sparse checkout. Sparse checkout support in 'git
 read-tree' and similar commands is disabled by default. You need to
 turn `core.sparseCheckout` on in order to have sparse checkout
 support.


 SEE ALSO
 --------
 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
 linkgit:gitignore[5]

 GIT
 ---
 Part of the linkgit:git[1] suite
	git-read-tree(1)
	================

	NAME
	----
	git-read-tree - Reads tree information into the index


	SYNOPSIS
	--------
	[verse]
	'git read-tree' [[-m [--trivial] [--aggressive] \| --reset \| --prefix=<prefix>]
	[-u [--exclude-per-directory=<gitignore>] \| -i]]
	[--index-output=<file>] [--no-sparse-checkout]
	(--empty \| <tree-ish1> [<tree-ish2> [<tree-ish3>]])


	DESCRIPTION
	-----------
	Reads the tree information given by <tree-ish> into the index,
	but does not actually update any of the files it "caches". (see:
	linkgit:git-checkout-index[1])

	Optionally, it can merge a tree into the index, perform a
	fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
	flag. When used with `-m`, the `-u` flag causes it to also update
	the files in the work tree with the result of the merge.

	Trivial merges are done by 'git read-tree' itself. Only conflicting paths
	will be in unmerged state when 'git read-tree' returns.

	OPTIONS
	-------
	-m::
	Perform a merge, not just a read. The command will
	refuse to run if your index file has unmerged entries,
	indicating that you have not finished previous merge you
	started.

	--reset::
	Same as -m, except that unmerged entries are discarded
	instead of failing.

	-u::
	After a successful merge, update the files in the work
	tree with the result of the merge.

	-i::
	Usually a merge requires the index file as well as the
	files in the working tree to be up to date with the
	current head commit, in order not to lose local
	changes. This flag disables the check with the working
	tree and is meant to be used when creating a merge of
	trees that are not directly related to the current
	working tree status into a temporary index file.

	-n::
	--dry-run::
	Check if the command would error out, without updating the index
	nor the files in the working tree for real.

	-v::
	Show the progress of checking files out.

	--trivial::
	Restrict three-way merge by 'git read-tree' to happen
	only if there is no file-level merging required, instead
	of resolving merge for trivial cases and leaving
	conflicting files unresolved in the index.

	--aggressive::
	Usually a three-way merge by 'git read-tree' resolves
	the merge for really trivial cases and leaves other
	cases unresolved in the index, so that porcelains can
	implement different merge policies. This flag makes the
	command resolve a few more cases internally:
	+
	* when one side removes a path and the other side leaves the path
	unmodified. The resolution is to remove that path.
	* when both sides remove a path. The resolution is to remove that path.
	* when both sides add a path identically. The resolution
	is to add that path.

	--prefix=<prefix>/::
	Keep the current index contents, and read the contents
	of the named tree-ish under the directory at `<prefix>`.
	The command will refuse to overwrite entries that already
	existed in the original index file. Note that the `<prefix>/`
	value must end with a slash.

	--exclude-per-directory=<gitignore>::
	When running the command with `-u` and `-m` options, the
	merge result may need to overwrite paths that are not
	tracked in the current branch. The command usually
	refuses to proceed with the merge to avoid losing such a
	path. However this safety valve sometimes gets in the
	way. For example, it often happens that the other
	branch added a file that used to be a generated file in
	your branch, and the safety valve triggers when you try
	to switch to that branch after you ran `make` but before
	running `make clean` to remove the generated file. This
	option tells the command to read per-directory exclude
	file (usually '.gitignore') and allows such an untracked
	but explicitly ignored file to be overwritten.

	--index-output=<file>::
	Instead of writing the results out to `$GIT_INDEX_FILE`,
	write the resulting index in the named file. While the
	command is operating, the original index file is locked
	with the same mechanism as usual. The file must allow
	to be rename(2)ed into from a temporary file that is
	created next to the usual index file; typically this
	means it needs to be on the same filesystem as the index
	file itself, and you need write permission to the
	directories the index file and index output file are
	located in.

	--no-sparse-checkout::
	Disable sparse checkout support even if `core.sparseCheckout`
	is true.

	--empty::
	Instead of reading tree object(s) into the index, just empty
	it.

	<tree-ish#>::
	The id of the tree object(s) to be read/merged.


	Merging
	-------
	If `-m` is specified, 'git read-tree' can perform 3 kinds of
	merge, a single tree merge if only 1 tree is given, a
	fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
	provided.


	Single Tree Merge
	~~~~~~~~~~~~~~~~~
	If only 1 tree is specified, 'git read-tree' operates as if the user did not
	specify `-m`, except that if the original index has an entry for a
	given pathname, and the contents of the path match with the tree
	being read, the stat info from the index is used. (In other words, the
	index's stat()s take precedence over the merged tree's).

	That means that if you do a `git read-tree -m <newtree>` followed by a
	`git checkout-index -f -u -a`, the 'git checkout-index' only checks out
	the stuff that really changed.

	This is used to avoid unnecessary false hits when 'git diff-files' is
	run after 'git read-tree'.


	Two Tree Merge
	~~~~~~~~~~~~~~

	Typically, this is invoked as `git read-tree -m $H $M`, where $H
	is the head commit of the current repository, and $M is the head
	of a foreign tree, which is simply ahead of $H (i.e. we are in a
	fast-forward situation).

	When two trees are specified, the user is telling 'git read-tree'
	the following:

	1. The current index and work tree is derived from $H, but
	the user may have local changes in them since $H.

	2. The user wants to fast-forward to $M.

	In this case, the `git read-tree -m $H $M` command makes sure
	that no local change is lost as the result of this "merge".
	Here are the "carry forward" rules, where "I" denotes the index,
	"clean" means that index and work tree coincide, and "exists"/"nothing"
	refer to the presence of a path in the specified commit:

	I H M Result
	-------------------------------------------------------
	0 nothing nothing nothing (does not happen)
	1 nothing nothing exists use M
	2 nothing exists nothing remove path from index
	3 nothing exists exists, use M if "initial checkout",
	H == M keep index otherwise
	exists, fail
	H != M

	clean I==H I==M
	------------------
	4 yes N/A N/A nothing nothing keep index
	5 no N/A N/A nothing nothing keep index

	6 yes N/A yes nothing exists keep index
	7 no N/A yes nothing exists keep index
	8 yes N/A no nothing exists fail
	9 no N/A no nothing exists fail

	10 yes yes N/A exists nothing remove path from index
	11 no yes N/A exists nothing fail
	12 yes no N/A exists nothing fail
	13 no no N/A exists nothing fail

	clean (H==M)
	------
	14 yes exists exists keep index
	15 no exists exists keep index

	clean I==H I==M (H!=M)
	------------------
	16 yes no no exists exists fail
	17 no no no exists exists fail
	18 yes no yes exists exists keep index
	19 no no yes exists exists keep index
	20 yes yes no exists exists use M
	21 no yes no exists exists fail

	In all "keep index" cases, the index entry stays as in the
	original index file. If the entry is not up to date,
	'git read-tree' keeps the copy in the work tree intact when
	operating under the -u flag.

	When this form of 'git read-tree' returns successfully, you can
	see which of the "local changes" that you made were carried forward by running
	`git diff-index --cached $M`. Note that this does not
	necessarily match what `git diff-index --cached $H` would have
	produced before such a two tree merge. This is because of cases
	18 and 19 --- if you already had the changes in $M (e.g. maybe
	you picked it up via e-mail in a patch form), `git diff-index
	--cached $H` would have told you about the change before this
	merge, but it would not show in `git diff-index --cached $M`
	output after the two-tree merge.

	Case 3 is slightly tricky and needs explanation. The result from this
	rule logically should be to remove the path if the user staged the removal
	of the path and then switching to a new branch. That however will prevent
	the initial checkout from happening, so the rule is modified to use M (new
	tree) only when the content of the index is empty. Otherwise the removal
	of the path is kept as long as $H and $M are the same.

	3-Way Merge
	~~~~~~~~~~~
	Each "index" entry has two bits worth of "stage" state. stage 0 is the
	normal one, and is the only one you'd see in any kind of normal use.

	However, when you do 'git read-tree' with three trees, the "stage"
	starts out at 1.

	This means that you can do

	----------------
	$ git read-tree -m <tree1> <tree2> <tree3>
	----------------

	and you will end up with an index with all of the <tree1> entries in
	"stage1", all of the <tree2> entries in "stage2" and all of the
	<tree3> entries in "stage3". When performing a merge of another
	branch into the current branch, we use the common ancestor tree
	as <tree1>, the current branch head as <tree2>, and the other
	branch head as <tree3>.

	Furthermore, 'git read-tree' has special-case logic that says: if you see
	a file that matches in all respects in the following states, it
	"collapses" back to "stage0":

	- stage 2 and 3 are the same; take one or the other (it makes no
	difference - the same work has been done on our branch in
	stage 2 and their branch in stage 3)

	- stage 1 and stage 2 are the same and stage 3 is different; take
	stage 3 (our branch in stage 2 did not do anything since the
	ancestor in stage 1 while their branch in stage 3 worked on
	it)

	- stage 1 and stage 3 are the same and stage 2 is different take
	stage 2 (we did something while they did nothing)

	The 'git write-tree' command refuses to write a nonsensical tree, and it
	will complain about unmerged entries if it sees a single entry that is not
	stage 0.

	OK, this all sounds like a collection of totally nonsensical rules,
	but it's actually exactly what you want in order to do a fast
	merge. The different stages represent the "result tree" (stage 0, aka
	"merged"), the original tree (stage 1, aka "orig"), and the two trees
	you are trying to merge (stage 2 and 3 respectively).

	The order of stages 1, 2 and 3 (hence the order of three
	<tree-ish> command line arguments) are significant when you
	start a 3-way merge with an index file that is already
	populated. Here is an outline of how the algorithm works:

	- if a file exists in identical format in all three trees, it will
	automatically collapse to "merged" state by 'git read-tree'.

	- a file that has _any_ difference what-so-ever in the three trees
	will stay as separate entries in the index. It's up to "porcelain
	policy" to determine how to remove the non-0 stages, and insert a
	merged version.

	- the index file saves and restores with all this information, so you
	can merge things incrementally, but as long as it has entries in
	stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
	now the merge algorithm ends up being really simple:

	* you walk the index in order, and ignore all entries of stage 0,
	since they've already been done.

	* if you find a "stage1", but no matching "stage2" or "stage3", you
	know it's been removed from both trees (it only existed in the
	original tree), and you remove that entry.

	* if you find a matching "stage2" and "stage3" tree, you remove one
	of them, and turn the other into a "stage0" entry. Remove any
	matching "stage1" entry if it exists too. .. all the normal
	trivial rules ..

	You would normally use 'git merge-index' with supplied
	'git merge-one-file' to do this last step. The script updates
	the files in the working tree as it merges each path and at the
	end of a successful merge.

	When you start a 3-way merge with an index file that is already
	populated, it is assumed that it represents the state of the
	files in your work tree, and you can even have files with
	changes unrecorded in the index file. It is further assumed
	that this state is "derived" from the stage 2 tree. The 3-way
	merge refuses to run if it finds an entry in the original index
	file that does not match stage 2.

	This is done to prevent you from losing your work-in-progress
	changes, and mixing your random changes in an unrelated merge
	commit. To illustrate, suppose you start from what has been
	committed last to your repository:

	----------------
	$ JC=`git rev-parse --verify "HEAD^0"`
	$ git checkout-index -f -u -a $JC
	----------------

	You do random edits, without running 'git update-index'. And then
	you notice that the tip of your "upstream" tree has advanced
	since you pulled from him:

	----------------
	$ git fetch git://.... linus
	$ LT=`git rev-parse FETCH_HEAD`
	----------------

	Your work tree is still based on your HEAD ($JC), but you have
	some edits since. Three-way merge makes sure that you have not
	added or modified index entries since $JC, and if you haven't,
	then does the right thing. So with the following sequence:

	----------------
	$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
	$ git merge-index git-merge-one-file -a
	$ echo "Merge with Linus" \| \
	git commit-tree `git write-tree` -p $JC -p $LT
	----------------

	what you would commit is a pure merge between $JC and $LT without
	your work-in-progress changes, and your work tree would be
	updated to the result of the merge.

	However, if you have local changes in the working tree that
	would be overwritten by this merge, 'git read-tree' will refuse
	to run to prevent your changes from being lost.

	In other words, there is no need to worry about what exists only
	in the working tree. When you have local changes in a part of
	the project that is not involved in the merge, your changes do
	not interfere with the merge, and are kept intact. When they
	do interfere, the merge does not even start ('git read-tree'
	complains loudly and fails without modifying anything). In such
	a case, you can simply continue doing what you were in the
	middle of doing, and when your working tree is ready (i.e. you
	have finished your work-in-progress), attempt the merge again.


	Sparse checkout
	---------------

	"Sparse checkout" allows populating the working directory sparsely.
	It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
	Git whether a file in the working directory is worth looking at.

	'git read-tree' and other merge-based commands ('git merge', 'git
	checkout'...) can help maintaining the skip-worktree bitmap and working
	directory update. `$GIT_DIR/info/sparse-checkout` is used to
	define the skip-worktree reference bitmap. When 'git read-tree' needs
	to update the working directory, it resets the skip-worktree bit in the index
	based on this file, which uses the same syntax as .gitignore files.
	If an entry matches a pattern in this file, skip-worktree will not be
	set on that entry. Otherwise, skip-worktree will be set.

	Then it compares the new skip-worktree value with the previous one. If
	skip-worktree turns from set to unset, it will add the corresponding
	file back. If it turns from unset to set, that file will be removed.

	While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
	files are in, you can also specify what files are _not_ in, using
	negate patterns. For example, to remove the file `unwanted`:

	----------------
	/*
	!unwanted
	----------------

	Another tricky thing is fully repopulating the working directory when you
	no longer want sparse checkout. You cannot just disable "sparse
	checkout" because skip-worktree bits are still in the index and your working
	directory is still sparsely populated. You should re-populate the working
	directory with the `$GIT_DIR/info/sparse-checkout` file content as
	follows:

	----------------
	/*
	----------------

	Then you can disable sparse checkout. Sparse checkout support in 'git
	read-tree' and similar commands is disabled by default. You need to
	turn `core.sparseCheckout` on in order to have sparse checkout
	support.


	SEE ALSO
	--------
	linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
	linkgit:gitignore[5]

	GIT
	---
	Part of the linkgit:git[1] suite