repack: add delta-islands support

repack: add delta-islands support

Implement simple support for --delta-islands option and
repack.useDeltaIslands config variable in git repack.

This allows users to setup delta islands in their config and
get the benefit of less disk usage while cloning and fetching
is still quite fast and not much more CPU intensive.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>