Let ls-files strip trailing slashes in submodules' paths

Tab completion makes it easy to add a trailing slash to a submodule path.
As it is completely clear what the user actually wanted to say, be nice
and strip that slash at the end.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -234,4 +234,10 @@ test_expect_success 'gracefully add submodule with a trailing slash' '
 
 '
 
+test_expect_success 'ls-files gracefully handles trailing slash' '
+
+	test "init" = "$(git ls-files init/)"
+
+'
+
 test_done