localrepo: avoid unnecessary conversion from node to rev

changelog.heads() first calls headrevs then converts them to nodes.
localrepo.heads() then sorts them using self.changelog.rev function and makes
useless conversion back to revs. Instead let's call changelog.headrevs() from
localrepo.heads(), sort the output and then convert to nodes. Because headrevs
does not support start parameter this optimization only works if start is None.

mercurial/localrepo.py

@@ -1852,6 +1852,10 b' class localrepository(object):'
                                   listsubrepos)
 
     def heads(self, start=None):
+        if start is None:
+            headrevs = sorted(self.changelog.headrevs(), reverse=True)
+            return [self.changelog.node(rev) for rev in headrevs]
+
         heads = self.changelog.heads(start)
         # sort the output in rev descending order
         return sorted(heads, key=self.changelog.rev, reverse=True)