branchmap: issue a single call to `ancestors` for all heads

There is no reason to make multiple calls. This provides a massive speedup for
repo with a lot of heads.

On a strongly headed repo this gives humble speedup in simple case:

    from 8.1097 to 5.1051

And massive speedup in other case:

    from 7.8787 to 0.1984

mercurial/branchmap.py

@@ -252,14 +252,10 b' class branchcache(dict):'
             newheadrevs.sort()
             bheadset.update(newheadrevs)
 
-            # This loop prunes out two kinds of heads - heads that are
-            # superseded by a head in newheadrevs, and newheadrevs that are not
-            # heads because an existing head is their descendant.
-            while newheadrevs:
-                latest = newheadrevs.pop()
-                if latest not in bheadset:
-                    continue
-                ancestors = set(cl.ancestors([latest], min(bheadset)))
+            # This prunes out two kinds of heads - heads that are superseded by
+            # a head in newheadrevs, and newheadrevs that are not heads because
+            # an existing head is their descendant.
+            ancestors = set(cl.ancestors(newheadrevs, min(bheadset)))
                 bheadset -= ancestors
             bheadrevs = sorted(bheadset)
             self[branch] = [cl.node(rev) for rev in bheadrevs]