revset: use parentsets.min in _children

As stated in the comment, using the smartset 'min' will give more opportunity to
be smart. It give a small but significant boost to the performance. Most of the
time is still spend doing the actual computation but at least we can scrap some
performance when it makes sense.

revset  #0: roots(0:tip)
   plain
0) 0.046600
1) 0.044109  94%

mercurial/revset.py

@@ -622,9 +622,7 b' def _children(repo, narrow, parentset):'
         return baseset()
     cs = set()
     pr = repo.changelog.parentrevs
-    # XXX this should be 'parentset.min()' assuming 'parentset' is a smartset
-    # (and if it is not, it should.)
-    minrev = min(parentset)
+    minrev = parentset.min()
     for r in narrow:
         if r <= minrev:
             continue