revset: use `subset &` in `follow`

This takes advantage of the `fullreposet` smartness.


revset  #0: follow(COPYING)
0) wall 0.002446 comb 0.000000 user 0.000000 sys 0.000000 (best of 735)
1) wall 0.000331 comb 0.000000 user 0.000000 sys 0.000000 (best of 5672)

mercurial/revset.py

@@ -834,7 +834,7 b' def _follow(repo, subset, x, name, follo'
     else:
         s = _revancestors(repo, baseset([c.rev()]), followfirst)
 
-    return subset.filter(s.__contains__)
+    return subset & s
 
 def follow(repo, subset, x):
     """``follow([file])``