submodule summary: do not shift a non-existent positional variable

When "git submodule summary" is run without any argument, we default to
compare the state of index with the HEAD, but tried to shift out $1 that
does not exist (and worse yet, we didn't use it).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -556,7 +556,7 @@ cmd_summary() {
 	if rev=$(git rev-parse -q --verify --default HEAD ${1+"$1"})
 	then
 		head=$rev
-		shift
+		test $# = 0 || shift
 	elif test -z "$1" -o "$1" = "HEAD"
 	then
 		return