Add -e/--exclude to git-clean.

Add -e/--exclude to git-clean.

With the -e/--exclude option for git-clean, a user can specify files
that they haven't yet told git about, but either need for a short amount
of time or plan to tell git about them later. This allows one to still
use git-clean while these files are around without losing data.

Signed-off-by: Jared Hance <jaredhance@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>