merge-ort: write $GIT_DIR/AUTO_MERGE whenever we hit a conflict

There are a variety of questions users might ask while resolving
conflicts:
  * What changes have been made since the previous (first) parent?
  * What changes are staged?
  * What is still unstaged? (or what is still conflicted?)
  * What changes did I make to resolve conflicts so far?
The first three of these have simple answers:
  * git diff HEAD
  * git diff --cached
  * git diff
There was no way to answer the final question previously.  Adding one
is trivial in merge-ort, since it works by creating a tree representing
what should be written to the working copy complete with conflict
markers.  Simply write that tree to .git/AUTO_MERGE, allowing users to
answer the fourth question with
  * git diff AUTO_MERGE

I avoided using a name like "MERGE_AUTO", because that would be
merge-specific (much like MERGE_HEAD, REBASE_HEAD, REVERT_HEAD,
CHERRY_PICK_HEAD) and I wanted a name that didn't change depending on
which type of operation the merge was part of.

Ensure that paths which clean out other temporary operation-specific
files (e.g. CHERRY_PICK_HEAD, MERGE_MSG, rebase-merge/ state directory)
also clean out this AUTO_MERGE file.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/sequencer.c b/sequencer.c
index d2332d3..472cdd8 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2096,6 +2096,7 @@ static int do_pick_commit(struct repository *r,
 		refs_delete_ref(get_main_ref_store(r), "", "CHERRY_PICK_HEAD",
 				NULL, 0);
 		unlink(git_path_merge_msg(r));
+		unlink(git_path_auto_merge(r));
 		fprintf(stderr,
 			_("dropping %s %s -- patch contents already upstream\n"),
 			oid_to_hex(&commit->object.oid), msg.subject);
@@ -2451,6 +2452,8 @@ void sequencer_post_commit_cleanup(struct repository *r, int verbose)
 		need_cleanup = 1;
 	}
 
+	unlink(git_path_auto_merge(r));
+
 	if (!need_cleanup)
 		return;
 
@@ -4111,6 +4114,7 @@ static int pick_commits(struct repository *r,
 			unlink(rebase_path_stopped_sha());
 			unlink(rebase_path_amend());
 			unlink(git_path_merge_head(r));
+			unlink(git_path_auto_merge(r));
 			delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
 
 			if (item->command == TODO_BREAK) {
@@ -4505,6 +4509,7 @@ static int commit_staged_changes(struct repository *r,
 		return error(_("could not commit staged changes."));
 	unlink(rebase_path_amend());
 	unlink(git_path_merge_head(r));
+	unlink(git_path_auto_merge(r));
 	if (final_fixup) {
 		unlink(rebase_path_fixup_msg());
 		unlink(rebase_path_squash_msg());