revset: make parents() O(number of parents)

Strip executes a revset like this:

max(parents(_intlist('1234\x001235')) - _intlist('1234\x001235'))

Previously the parents() revset would do 'subset & parents' which iterates over
each item in the subset and checks if it's in parents.  subset is usually the
entire repo (a spanset) so this takes a while.

Reversing the parameters to be 'parents & subset' means the operation becomes
O(number of parents) instead of O(size of repo). It also means the result gets
evaluated immediately (since parents isn't a lazy set), but I think this is a
win in most scenarios.

This shaves 0.3 seconds off strip (amend/histedit/rebase/etc) for large repositories.

revset  #0: parents(20000)
0) obsolete feature not enabled but 54243 markers found!
! wall 0.006256 comb 0.010000 user 0.010000 sys 0.000000 (best of 289)
1) obsolete feature not enabled but 54243 markers found!
! wall 0.000391 comb 0.000000 user 0.000000 sys 0.000000 (best of 4323)

contrib/revsetbenchmarks.txt

@@ -23,4 +23,5 draft()
 max(::(tip~20) - obsolete())
 roots((0:tip)::)
 (not public() - obsolete())
+parents(20000)
 (20000::) - (20000)

mercurial/revset.py

@@ -1236,7 +1236,7 def parents(repo, subset, x):
     cl = repo.changelog
     for r in getset(repo, spanset(repo), x):
         ps.update(cl.parentrevs(r))
-    return subset & ps
+    return baseset(ps) & subset
 
 def parentspec(repo, subset, x, n):
     """``set^0``

tests/test-revset.t

@@ -567,10 +567,10 test subtracting something from an addse
 test intersecting something with an addset
 
   $ log 'parents(outgoing() or removes(a))'
+  8
   1
   4
   5
-  8
 
 check that conversion to only works
   $ try --optimize '::3 - ::1'