'git-ls-files --ignored --directories' hides empty directories even though
--no-empty-directory was not specified.

Treat the DIR_HIDE_EMPTY_DIRECTORIES flag independently from
DIR_SHOW_IGNORED to make all git-ls-files options work as expected.
Signed-off-by: NKarsten Blees <blees@dcon.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>