dir.c: git-status --ignored: don't drop ignored directories

'git-status --ignored' drops ignored directories if they contain untracked
files in an untracked sub directory.

Fix it by getting exact (recursive) excluded status in treat_directory.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>