t3905: show failure to ignore sub-repo

"git stash" used to ignore sub-repositories until 6e773527b6
(sparse-index: convert from full to sparse, 2021-03-30).  Add a test
that demonstrates this regression.

Reported-by: Robert Leftwich <robert@gitpod.io>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3905-stash-include-untracked.sh

@@ -405,4 +405,10 @@ test_expect_success 'stash show --include-untracked errors on duplicate files' '
 	test_i18ngrep "worktree and untracked commit have duplicate entries: tracked" err
 '
 
+test_expect_failure 'stash -u ignores sub-repository' '
+	test_when_finished "rm -rf sub-repo" &&
+	git init sub-repo &&
+	git stash -u
+'
+
 test_done