git-stash: allow more descriptive reminder message when saving

This allows you to say:

	$ git stash starting to implement X

while creating a stash, and the resulting "stash list entry
would read as:

	$ git stash list
        stash@{0}: On master: starting to implement X

instead of the default message which talks about the commit the
stash happens to be based on (hence does not have much to do
with what the stashed change is trying to do).
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -23,6 +23,8 @@ clear_stash () {
 }
 
 save_stash () {
+	stash_msg="$1"
+
 	if no_changes
 	then
 		echo >&2 'No local changes to save'
@@ -67,13 +69,19 @@ save_stash () {
 		die "Cannot save the current worktree state"
 
 	# create the stash
-	w_commit=$(printf 'WIP on %s\n' "$msg" |
+	if test -z "$stash_msg"
+	then
+		stash_msg=$(printf 'WIP on %s' "$msg")
+	else
+		stash_msg=$(printf 'On %s: %s' "$branch" "$stash_msg")
+	fi
+	w_commit=$(printf '%s\n' "$stash_msg" |
 		git commit-tree $w_tree -p $b_commit -p $i_commit) ||
 		die "Cannot record working tree state"
 
-	git update-ref -m "WIP on $msg" $ref_stash $w_commit ||
+	git update-ref -m "$stash_msg" $ref_stash $w_commit ||
 		die "Cannot save the current status"
-	printf >&2 'Saved WIP on %s\n' "$msg"
+	printf >&2 'Saved "%s"\n' "$stash_msg"
 }
 
 have_stash () {
@@ -157,9 +165,14 @@ apply)
 clear)
 	clear_stash
 	;;
-save | '')
-	save_stash && git-reset --hard
+help | usage)
+	usage
 	;;
 *)
-	usage
+	if test $# -gt 0 && test "$1" = save
+	then
+		shift
+	fi
+	save_stash "$*" && git-reset --hard
+	;;
 esac