Support incremental backups to a remote server.

Support incremental backups to a remote server.

We now cache the server's packfile indexes locally, so we know which objects
he does and doesn't have.  That way we can send him a packfile with only the
ones he's missing.

cmd-split supports this now, but cmd-save still doesn't support remote
servers.

The -n option (set a ref correctly) doesn't work yet either.