Add delta-islands.{c,h}

Add delta-islands.{c,h}

Hosting providers that allow users to "fork" existing
repos want those forks to share as much disk space as
possible.

Alternates are an existing solution to keep all the
objects from all the forks into a unique central repo,
but this can have some drawbacks. Especially when
packing the central repo, deltas will be created
between objects from different forks.

This can make cloning or fetching a fork much slower
and much more CPU intensive as Git might have to
compute new deltas for many objects to avoid sending
objects from a different fork.

Because the inefficiency primarily arises when an
object is deltified against another object that does
not exist in the same fork, we partition objects into
sets that appear in the same fork, and define
"delta islands". When finding delta base, we do not
allow an object outside the same island to be
considered as its base.

So "delta islands" is a way to store objects from
different forks in the same repo and packfile without
having deltas between objects from different forks.

This patch implements the delta islands mechanism in
"delta-islands.{c,h}", but does not yet make use of it.

A few new fields are added in 'struct object_entry'
in "pack-objects.h" though.

The documentation will follow in a patch that actually
uses delta islands in "builtin/pack-objects.c".

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>