submodule: warn about non-submodules

Earlier, when you called

        git submodule some/bogus/path

Git would silently ignore the path, without warning the user about the
likely mistake.  Now it does.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -59,7 +59,7 @@ resolve_relative_url ()
 #
 module_list()
 {
-	git ls-files --stage -- "$@" | grep '^160000 '
+	git ls-files --error-unmatch --stage -- "$@" | grep '^160000 '
 }
 
 #

t/t7400-submodule-basic.sh

@@ -240,4 +240,11 @@ test_expect_success 'ls-files gracefully handles trailing slash' '
 
 '
 
+test_expect_success 'submodule <invalid-path> warns' '
+
+	git submodule no-such-submodule 2> output.err &&
+	grep "^error: .*no-such-submodule" output.err
+
+'
+
 test_done