Use ".have" advertisements during push to reduce what is transferred

Use ".have" advertisements during push to reduce what is transferred

If a remote peer has an alternate object database it might show
the refs of the alternate repositories by sending special ".have"
ref lines in the advertisement.  These can be used to compute a
better common base, avoiding uploading objects which the remote
side has physically available (and complete), but which aren't in
the ref space of the remote repository.

As most repositories have more than one ref, there can be more than
one ".have" line, and the same object might appear more than once.
Collecting these into a special "additionalHaves" set avoids them
from showing up in the advertised refs collection.

PackWriter already gracefully skips over remoteObjects that
we don't have locally.  We can simply include the entire set
of additionalHaves when we start creating a pack for the push
transfer and PackWriter will seek the best common base available.
Anything advertised that we lack will be silently skipped.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>