rev-branch-cache: add a function to generate a part The function is able to produce a rbc part consumed by the function introduced into previous changesets. More details on usage and impact in the next changesets.

revbranchcache: add a bundle2 handler for a rbc part We add the necessary bit to process a part containing rev-branch-cache information. The local rev branch cache is then updated with the received information. Computing branch cache can become a significant part of the time spent pulling.

revbranchcache: add a public function to update the data We want to exchange more cached data over the wire. To do so, we need a clean way to update the cache on the receiving ends.

httppeer: consolidate _requestbuilder assignments and document I collapsed multiple assignments because they don't appear to be necessary. We don't invoke the requestbuilder in anything called during __init__. So there's no reason to not define it earlier AFAICT. This seemingly useless attribute is actually an extension point. Document it as such. (A previous version of this patch removed the attribute because it appeared to just be an alias.) Differential Revision: https://phab.mercurial-scm.org/D2723