submodule add: test failure when url is not configured in superproject

This documents the current behavior (submodule add with the url set in the
superproject is already tested in t7403, t7406, t7407 and t7506).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -446,6 +446,13 @@ test_expect_success 'add should fail when path is used by an existing directory'
 	)
 '
 
+test_expect_success 'add should fail when path is relative but no url is set in the superproject' '
+	(
+		cd addtest &&
+		test_must_fail git submodule add ../repo relative
+	)
+'
+
 test_expect_success 'set up for relative path tests' '
 	mkdir reltest &&
 	(