revset-only: use `subset &` instead of filtering

The & version is more likely to be optimised.

only(.)
before) wall 0.003216 comb 0.000000 user 0.000000 sys 0.000000 (best of 768)
after)  wall 0.001086 comb 0.000000 user 0.000000 sys 0.000000 (best of 2231)

only(default, stable)
before) wall 0.018469 comb 0.020000 user 0.020000 sys 0.000000 (best of 138)
after)  wall 0.015888 comb 0.010000 user 0.010000 sys 0.000000 (best of 156)

mercurial/revset.py

@@ -408,7 +408,7 b' def only(repo, subset, x):'
         exclude = getset(repo, spanset(repo), args[1])
 
     results = set(ancestormod.missingancestors(include, exclude, cl.parentrevs))
-    return filteredset(subset, results.__contains__)
+    return subset & results
 
 def bisect(repo, subset, x):
     """``bisect(string)``