revset: use `subset &` in `outgoing`

This should give us the same benefit as elsewhere. Result is simpler (and
"faster").

Outgoing is dominated by the discovery so no benchmark is provided.

mercurial/revset.py

@@ -1190,7 +1190,7 b' def outgoing(repo, subset, x):'
     repo.ui.popbuffer()
     cl = repo.changelog
     o = set([cl.rev(r) for r in outgoing.missing])
-    return subset.filter(o.__contains__)
+    return subset & o
 
 def p1(repo, subset, x):
     """``p1([set])``