revset: use `subset &` in `origin`

This takes advantage of the `fullreposet` smartness.

revset  #0: origin(tip)
0) wall 0.005353 comb 0.000000 user 0.000000 sys 0.000000 (best of 354)
1) wall 0.003080 comb 0.000000 user 0.000000 sys 0.000000 (best of 446)

mercurial/revset.py

@@ -1167,7 +1167,7 b' def origin(repo, subset, x):'
 
     o = set([_firstsrc(r) for r in args])
     o -= set([None])
-    return subset.filter(o.__contains__)
+    return subset & o
 
 def outgoing(repo, subset, x):
     """``outgoing([path])``