test-template-engine: deduplicate methods of custom template engine

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)

revbranchcache: disable the new part for narrow hg bundle The lack of some revisions confuses the new cache part. To simplify things, we disable it for now.