git-submodule: Fix typo 'url' which should be '$url'

Fix typo in 'test -z "url"' when checking whether a submodule url is
empty. "url" should be "$url".

Signed-off-by: Ping Yin <pkufranky@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -362,7 +362,7 @@ cmd_status()
 	do
 		name=$(module_name "$path") || exit
 		url=$(git config submodule."$name".url)
-		if test -z "url" || ! test -d "$path"/.git
+		if test -z "$url" || ! test -d "$path"/.git
 		then
 			say "-$sha1 $path"
 			continue;