pack-objects: do not reuse packfiles without --delta-base-offset

When we are sending a packfile to a remote, we currently try
to reuse a whole chunk of packfile without bothering to look
at the individual objects. This can make things like initial
clones much lighter on the server, as we can just dump the
packfile bytes.

However, it's possible that the other side cannot read our
packfile verbatim. For example, we may have objects stored
as OFS_DELTA, but the client is an antique version of git
that only understands REF_DELTA. We negotiate this
capability over the fetch protocol. A normal pack-objects
run will convert OFS_DELTA into REF_DELTA on the fly, but
the "reuse pack" code path never even looks at the objects.

This patch disables packfile reuse if the other side is
missing any capabilities that we might have used in the
on-disk pack. Right now the only one is OFS_DELTA, but we
may need to expand in the future (e.g., if packv4 introduces
new object types).

We could be more thorough and only disable reuse in this
case when we actually have an OFS_DELTA to send, but:

  1. We almost always will have one, since we prefer
     OFS_DELTA to REF_DELTA when possible. So this case
     would almost never come up.

  2. Looking through the objects defeats the purpose of the
     optimization, which is to do as little work as possible
     to get the bytes to the remote.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>