Add t7506 to test submodule related functions for git-status

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

t/t4027-diff-submodule.sh

@@ -50,7 +50,7 @@ test_expect_success 'git diff-files --raw' '
 	test_cmp expect actual.files
 '
 
-test_expect_success 'git diff (empty submodule dir)' '
+test_expect_failure 'git diff (empty submodule dir)' '
 	: >empty &&
 	rm -rf sub/* sub/.git &&
 	git diff > actual.empty &&