git-svn: make --repack work consistently between fetch and multi-fetch

git-svn: make --repack work consistently between fetch and multi-fetch

Since fetch reforks itself at most every 1000 revisions, we
need to update the counter in the parent process to have a
working count if we set our repack interval to be > ~1000
revisions.  multi-fetch has always done this correctly
because of an extra process; now fetch uses the extra process;
as well.

While we're at it, only compile the $sha1 regex that checks for
repacking once.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>