stash: recognize "--help" for subcommands

If you run "git stash --help", you get the help for stash
(this magic is done by the git wrapper itself). But if you
run "git stash drop --help", you get an error. We
cannot show help specific to "stash drop", of course, but we
can at least give the user the normal stash manpage.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -219,6 +219,9 @@ save_stash () {
 		-a|--all)
 			untracked=all
 			;;
+		--help)
+			show_help
+			;;
 		--)
 			shift
 			break
@@ -307,6 +310,11 @@ show_stash () {
 	git diff ${FLAGS:---stat} $b_commit $w_commit
 }
 
+show_help () {
+	exec git help stash
+	exit 1
+}
+
 #
 # Parses the remaining options looking for flags and
 # at most one revision defaulting to ${ref_stash}@{0}
@@ -373,6 +381,9 @@ parse_flags_and_rev()
 			--index)
 				INDEX_OPTION=--index
 			;;
+			--help)
+				show_help
+			;;
 			-*)
 				test "$ALLOW_UNKNOWN_FLAGS" = t ||
 					die "$(eval_gettext "unknown option: \$opt")"