Teach fast-import how to sit quietly in the corner. Often users will be running fast-import from within a larger frontend process, and this may be a frequent periodic tool such as a future edition of `git-svn fetch`. We don't want to bombard users with our large stats output if they won't be interested in it, so `--quiet` is now an option to make gfi be more silent. Signed-off-by: Shawn O. Pearce <spearce@spearce.org>