discovery: simplify the building of the children mapping

Since we only care about the revisions inside the set we are sampling, we can
use simpler code (and probably sightly faster).

mercurial/setdiscovery.py

@@ -219,13 +219,13 b' class partialdiscovery(object):'
         for rev in sorted(revs):
             # Always ensure revision has an entry so we don't need to worry
             # about missing keys.
-            children.setdefault(rev, [])
-
+            children[rev] = []
             for prev in parentrevs(rev):
                 if prev == nullrev:
                     continue
-
-                children.setdefault(prev, []).append(rev)
+                c = children.get(prev)
+                if c is not None:
+                    c.append(rev)
 
         _updatesample(revs, revsroots, sample, children.__getitem__)
         assert sample