revset.children: ignore rev numbers that are too low

This replaces unnecessary parentrevs() calls with calculating min(parentset).
Even though the min operation is O(size of parentset), since parentrevs is
relatively expensive, this tradeoff almost always works in our favour. In a
repository with over 400,000 changesets, hg perfrevset "children(X)" takes:

       Set X       Before    After
       -1           0.51s    0.06s
    -1000:          0.55s    0.08s
   -10000:          0.56s    0.10s
  -100000:          0.60s    0.25s
  -100000:-99000    0.55s    0.19s
        0:100000    0.60s    0.61s
      all()         0.72s    0.74s

The relative performance is similar for Mercurial's own repository -- several
times faster in most cases, slightly slower for revisions close to 0 and
all().

mercurial/revset.py

@@ -488,8 +488,13 b' def checkstatus(repo, subset, pat, field'
 
 def _children(repo, narrow, parentset):
     cs = set()
+    if not parentset:
+        return cs
     pr = repo.changelog.parentrevs
+    minrev = min(parentset)
     for r in narrow:
+        if r <= minrev:
+            continue
         for p in pr(r):
             if p in parentset:
                 cs.add(r)