revbranchcache: advertise and use 'rbc' exchange capability The feature is now advertised and use. Updating the branchmap cache can be very expensive (up to minutes on large repository) and fetching revision branch data is about 80% of that. Exchanging the rev branch cache over the wire really help to recover from branch map invalidation. (There is a good chance other in flight chance would conflict on test-http-bad-server.t and other. So here is a small note to help update that test again: capabilities=19bytes, part-107bytes)