Don't always require working tree for git-rm

Don't always require working tree for git-rm

This allows to do git rm --cached -r directory, instead of
git ls-files -z directory | git update-index --remove -z --stdin.
This can be particularly useful for git-filter-branch users.

Signed-off-by: Mike Hommey <mh@glandium.org>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>