strip: incrementally update the branchheads cache after a strip This function augments strip to incrementally update the branchheads cache rather than recompute it from scratch. This speeds up the performance of strip and rebase on repos with long history. The performance optimization only happens if the revisions stripped are all on the same branch and the parents of the stripped revisions are also on that same branch. This adds a few test cases, particularly one that reproduces the extra heads that mpm observed.

localrepo: convert _updatebranchcache from nodespace to revspace _updatebranchcache used to use revlog.reachable. After the switch to revlog.ancestors, we can now clean it up a bit and switch the algorithm from nodes to revs.