revset-_ancestor: use & instead of filter

The & operation is more likely optimised.

::10
before) wall 0.028189 comb 0.030000 user 0.030000 sys 0.000000 (best of 100)
after)  wall 0.001050 comb 0.000000 user 0.000000 sys 0.000000 (best of 2326)

::tip
before) wall 0.081132 comb 0.080000 user 0.080000 sys 0.000000 (best of 100)
after)  wall 0.055418 comb 0.050000 user 0.050000 sys 0.000000 (best of 100)

mercurial/revset.py

@@ -347,7 +347,7 b' def _ancestors(repo, subset, x, followfi'
     if not heads:
         return baseset()
     s = _revancestors(repo, heads, followfirst)
-    return subset.filter(s.__contains__)
+    return subset & s
 
 def ancestors(repo, subset, x):
     """``ancestors(set)``