revert: bring back usage of `subset & ps` in `parents`

Changeset 95af98616aa7 switched the order of the operand of the "&" computation
to work around an issue from repo-wide spanset. The need for a workaround has been
alleviated by the introduction of `fullreposet`. So we restore it to normal.

The benchmark shows no significant changes as expected.

We also revert the bogus test change introduced by 95af98616aa7. The order is
actually important.

mercurial/revset.py

@@ -1242,7 +1242,7 b' def parents(repo, subset, x):'
         for r in getset(repo, spanset(repo), x):
             ps.update(cl.parentrevs(r))
     ps -= set([node.nullrev])
-    return baseset(ps) & subset
+    return subset & ps
 
 def parentspec(repo, subset, x, n):
     """``set^0``

tests/test-revset.t

@@ -567,10 +567,10 b' 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'