revset: use `subset &` in `ancestorspec`

This takes advantage of the `fullreposet` smartness.


revset  #0: tip~25
0) wall 0.004800 comb 0.010000 user 0.010000 sys 0.000000 (best of 259)
1) wall 0.002475 comb 0.000000 user 0.000000 sys 0.000000 (best of 717)

mercurial/revset.py

@@ -376,7 +376,7 b' def ancestorspec(repo, subset, x, n):'
         for i in range(n):
             r = cl.parentrevs(r)[0]
         ps.add(r)
-    return subset.filter(ps.__contains__)
+    return subset & ps
 
 def author(repo, subset, x):
     """``author(string)``