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

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

Signed-off-by: Ping Yin <042bd97809d88aa56810834a9e2526d4839ab650@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git