Teach fast-import how to sit quietly in the corner.

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>