stash: avoid recursive hard reset on submodules

stash: avoid recursive hard reset on submodules

git stash push does not recursively stash submodules, but if
submodule.recurse is set, it may recursively reset --hard them. Having
only the destructive action recurse is likely to be surprising
behaviour, and unlikely to be desirable, so the easiest fix should be to
ensure that the call to git reset --hard never recurses into submodules.

This matches the behavior of check_changes_tracked_files, which ignores
submodules.

Signed-off-by: Jakob Jarmar <jakob@jarmar.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>