Let "git svn" run "git gc --auto" occasionally

Let "git svn" run "git gc --auto" occasionally

Let "git svn" run "git gc --auto" every 1000 imported commits to
reduce the number of loose objects.

To handle the common use case of frequent imports, where each
invocation typically fetches much less than 1000 commits, also run gc
unconditionally at the end of the import.

"1000" is the same number that was used by default when we called
git-repack. It isn't necessarily still the best choice.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>