git status: skip empty directories, and add -u to show all untracked files

git status: skip empty directories, and add -u to show all untracked files

By default, we use --others --directory to show uninteresting
directories (to get user's attention) without their contents (to
unclutter output).  Showing empty directories do not make sense,
so pass --no-empty-directory when we do so.

Giving -u (or --untracked) disables this uncluttering to let the
user get all untracked files.

Signed-off-by: Junio C Hamano <junkio@cox.net>