cvsimport: skip commits that are too recent

cvsimport: skip commits that are too recent

With this patch, cvsimport will skip commits made
in the last 10 minutes. The recent-ness test is of
5 minutes + cvsps fuzz window (5 minutes default).

When working with a CVS repository that is in use,
importing commits that are too recent can lead to
partially incorrect trees. This is mainly due to

 - Commits that are within the cvsps fuzz window may later
   be found to have affected more files.

 - When performing incremental imports, clock drift between
   the systems may lead to skipped commits.

This commit helps keep incremental imports of in-use
CVS repositories sane.

Signed-off-by: Martin Langhoff <martin@catalyst.net.nz>
Signed-off-by: Junio C Hamano <junkio@cox.net>