t4027: test diff for submodule with empty directory

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

t/t4027-diff-submodule.sh

@@ -50,4 +50,11 @@ test_expect_success 'git diff-files --raw' '
 	test_cmp expect actual.files
 '
 
+test_expect_success 'git diff (empty submodule dir)' '
+	: >empty &&
+	rm -rf sub/* sub/.git &&
+	git diff > actual.empty &&
+	test_cmp empty actual.empty
+'
+
 test_done