revset: use `subset &` in bare `p1()`

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

revset  #0: p1()
0) wall 0.003256 comb 0.010000 user 0.010000 sys 0.000000 (best of 527)
1) wall 0.000066 comb 0.000000 user 0.000000 sys 0.000000 (best of 23224)

mercurial/revset.py

@@ -1198,7 +1198,9 b' def p1(repo, subset, x):'
     """
     if x is None:
         p = repo[x].p1().rev()
-        return subset.filter(lambda r: r == p)
+        if p >= 0:
+            return subset & baseset([p])
+        return baseset([])
 
     ps = set()
     cl = repo.changelog