revbranchcache: add the necessary bit to send 'rbc' data over bundle2 Getbundle is now capable of sending rev-branch-cache information for the changesets it bundle. The data sent are mostly nodes so it is quite compact. The goal of the rev-branch-cache is to speed up branch map computation, especially when the branchmap gets invalidated so we send data for all exchanged changesets. In addition, computing the relevant heads to send in case of partial pulling would be challenging. The feature is still inactive since the capability is not advertised yet.