Stop displaying "Pack pack-$ID created." during git-gc

Stop displaying "Pack pack-$ID created." during git-gc

Discussion on the list tonight came to the conclusion that showing
the name of the packfile we just created during git-repack is not
a very useful message for any end-user.  For the really technical
folk who need to have the name of the newest packfile they can use
something such as `ls -t .git/objects/pack | head -2` to find the
most recently created packfile.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>