Optionally do not list empty directories in git-ls-files --others

Optionally do not list empty directories in git-ls-files --others

Without the --directory flag, git-ls-files wouldn't ever list directories,
producing no output for empty directories, which is good since they cannot
be added and they bear no content, even untracked one (if Git ever starts
tracking directories on their own, this should obviously change since the
content notion will change).

With the --directory flag however, git-ls-files would list even empty
directories. This may be good in some situations but sometimes you want to
prevent that. This patch adds a --no-empty-directory option which makes
git-ls-files omit empty directories.

Signed-off-by: Petr Baudis <pasky@suse.cz>