git-stash: fix "can't shift that many" with no arguments

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -133,7 +133,7 @@ apply_stash () {
 # Main command set
 case "$1" in
 list | '')
-	shift
+	test $# -gt 0 && shift
 	if test $# = 0
 	then
 		set x -n 10