git-ls-files: --exclude mechanism updates.

git-ls-files: --exclude mechanism updates.

Add --exclude-per-directory=<name> option that specifies a file
to contain exclude patterns local to that directory and its
subdirectories.  Update the exclusion logic to be able to say
"include files that match this more specific pattern, even
though later exclude patterns may match them".  Also enhances
that a pattern can contain '/' in which case fnmatch is called
with FNM_PATHNAME flag to match the entire path.

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