rebase: refactor common shell functions into their own file

The functions present in `git-legacy-rebase.sh` are used by the rebase
backends as they are implemented as shell script functions in the
`git-rebase--<backend>` files.

To make the `builtin/rebase.c` work, we have to provide support via
a Unix shell script snippet that uses these functions and so, we
want to use the rebase backends *directly* from the builtin rebase
without going through `git-legacy-rebase.sh`.

This commit extracts the functions to a separate file,
`git-rebase--common`, that will be read by `git-legacy-rebase.sh` and
by the shell script snippets which will be used extensively in the
following commits.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-legacy-rebase.sh b/git-legacy-rebase.sh
index 7973447..af2cdfe 100755
--- a/git-legacy-rebase.sh
+++ b/git-legacy-rebase.sh
@@ -57,12 +57,7 @@
 LF='
 '
 ok_to_skip_pre_rebase=
-resolvemsg="
-$(gettext 'Resolve all conflicts manually, mark them as resolved with
-"git add/rm <conflicted_files>", then run "git rebase --continue".
-You can instead skip this commit: run "git rebase --skip".
-To abort and get back to the state before "git rebase", run "git rebase --abort".')
-"
+
 squash_onto=
 unset onto
 unset restrict_revision
@@ -102,6 +97,7 @@
 true)	gpg_sign_opt=-S ;;
 *)	gpg_sign_opt= ;;
 esac
+. git-rebase--common
 
 read_basic_state () {
 	test -f "$state_dir/head-name" &&
@@ -132,67 +128,6 @@
 	}
 }
 
-write_basic_state () {
-	echo "$head_name" > "$state_dir"/head-name &&
-	echo "$onto" > "$state_dir"/onto &&
-	echo "$orig_head" > "$state_dir"/orig-head &&
-	echo "$GIT_QUIET" > "$state_dir"/quiet &&
-	test t = "$verbose" && : > "$state_dir"/verbose
-	test -n "$strategy" && echo "$strategy" > "$state_dir"/strategy
-	test -n "$strategy_opts" && echo "$strategy_opts" > \
-		"$state_dir"/strategy_opts
-	test -n "$allow_rerere_autoupdate" && echo "$allow_rerere_autoupdate" > \
-		"$state_dir"/allow_rerere_autoupdate
-	test -n "$gpg_sign_opt" && echo "$gpg_sign_opt" > "$state_dir"/gpg_sign_opt
-	test -n "$signoff" && echo "$signoff" >"$state_dir"/signoff
-}
-
-output () {
-	case "$verbose" in
-	'')
-		output=$("$@" 2>&1 )
-		status=$?
-		test $status != 0 && printf "%s\n" "$output"
-		return $status
-		;;
-	*)
-		"$@"
-		;;
-	esac
-}
-
-move_to_original_branch () {
-	case "$head_name" in
-	refs/*)
-		message="rebase finished: $head_name onto $onto"
-		git update-ref -m "$message" \
-			$head_name $(git rev-parse HEAD) $orig_head &&
-		git symbolic-ref \
-			-m "rebase finished: returning to $head_name" \
-			HEAD $head_name ||
-		die "$(eval_gettext "Could not move back to \$head_name")"
-		;;
-	esac
-}
-
-apply_autostash () {
-	if test -f "$state_dir/autostash"
-	then
-		stash_sha1=$(cat "$state_dir/autostash")
-		if git stash apply $stash_sha1 >/dev/null 2>&1
-		then
-			echo "$(gettext 'Applied autostash.')" >&2
-		else
-			git stash store -m "autostash" -q $stash_sha1 ||
-			die "$(eval_gettext "Cannot store \$stash_sha1")"
-			gettext 'Applying autostash resulted in conflicts.
-Your changes are safe in the stash.
-You can run "git stash pop" or "git stash drop" at any time.
-' >&2
-		fi
-	fi
-}
-
 finish_rebase () {
 	rm -f "$(git rev-parse --git-path REBASE_HEAD)"
 	apply_autostash &&