branch heads: optimise computation of branch head cache (issue1734)

The previous branch heads cache implementation iterated all ancestors
for each new revision in the repository, causing a massive slowdown on
cloning larger repositories.

mercurial/localrepo.py

@@ -473,9 +473,8 b' class localrepository(repo.repository):'
                 latest = newnodes.pop()
                 if latest not in bheads:
                     continue
-                reachable = set()
-                for bh in bheads:
-                    reachable |= self.changelog.reachable(latest, bh)
+                minbhrev = self[min([self[bh].rev() for bh in bheads])].node()
+                reachable = self.changelog.reachable(latest, minbhrev)
                 bheads = [b for b in bheads if b not in reachable]
                 newbheads.insert(0, latest)
             bheads.extend(newbheads)