revset: use `subset &` in `rev`

This takes advantage of the `fullreposet` smartness and yields a nice
speedup.

revset  #0: rev(25)
0) wall 0.005480 comb 0.000000 user 0.000000 sys 0.000000 (best of 305)
1) wall 0.000052 comb 0.000000 user 0.000000 sys 0.000000 (best of 21891)

mercurial/revset.py

@@ -1346,7 +1346,7 b' def rev(repo, subset, x):'
     except (TypeError, ValueError):
         # i18n: "rev" is a keyword
         raise error.ParseError(_("rev expects a number"))
-    return subset.filter(lambda r: r == l)
+    return subset & baseset([l])
 
 def matching(repo, subset, x):
     """``matching(revision [, field])``