pack-objects: add delta-islands support

pack-objects: add delta-islands support

Implement support for delta islands in git pack-objects
and document how delta islands work in
"Documentation/git-pack-objects.txt" and Documentation/config.txt.

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>