stash: implement "stash create"

This subcommand creates a stash from the current state and writes out the
resulting commit object ID to the standard output, without updating the
stash ref nor resetting the tree.  It is intended to be used by scripts
to temporarily rewind the working tree to a clean state.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -25,19 +25,13 @@ clear_stash () {
 	fi
 }
 
-save_stash () {
+create_stash () {
 	stash_msg="$1"
 
 	if no_changes
 	then
-		echo >&2 'No local changes to save'
 		exit 0
 	fi
-	test -f "$GIT_DIR/logs/$ref_stash" ||
-		clear_stash || die "Cannot initialize stash"
-
-	# Make sure the reflog for stash is kept.
-	: >>"$GIT_DIR/logs/$ref_stash"
 
 	# state of the base commit
 	if b_commit=$(git rev-parse --verify HEAD)
@@ -84,7 +78,20 @@ save_stash () {
 	w_commit=$(printf '%s\n' "$stash_msg" |
 		git commit-tree $w_tree -p $b_commit -p $i_commit) ||
 		die "Cannot record working tree state"
+}
 
+save_stash () {
+	stash_msg="$1"
+
+	if no_changes
+	then
+		echo >&2 'No local changes to save'
+		exit 0
+	fi
+	test -f "$GIT_DIR/logs/$ref_stash" ||
+		clear_stash || die "Cannot initialize stash"
+
+	create_stash "$stash_msg"
 	git update-ref -m "$stash_msg" $ref_stash $w_commit ||
 		die "Cannot save the current status"
 	printf >&2 'Saved "%s"\n' "$stash_msg"
@@ -202,6 +209,13 @@ apply)
 clear)
 	clear_stash
 	;;
+create)
+	if test $# -gt 0 && test "$1" = create
+	then
+		shift
+	fi
+	create_stash "$*" && echo "$w_commit"
+	;;
 help | usage)
 	usage
 	;;