branchmap: update rev-branch-cache incrementally Historically, the revision to branch mapping cache was updated on demand and shared via bundle2 to avoid the cost of rebuilding on first use. Use the new `register_changeset` callback and update rbc directly on every change. Make the transfer of the bundle part redundant, but keep it for the moment to avoid the test churn. Over all, "hg unbundle" for large bundles is less than 1.8% slower for different larger repositories and that seems to a reasonable trade off. Differential Revision: https://phab.mercurial-scm.org/D9781