discovery: use a lower level but faster way to retrieve parents

We already know that no revision in the undecided set are filtered, so we can
skip multiple checks and directly access lower level data.

In a private pathological case, this improves the timing from about 70 seconds
to about 50 seconds. There are other actions to be taken to improve that case,
however this gives an idea of the general overhead.

mercurial/setdiscovery.py

@@ -165,6 +165,12 b' class partialdiscovery(object):'
         # common.bases and all its ancestors
         return self._common.basesheads()
 
+    def _parentsgetter(self):
+        getrev = self._repo.changelog.index.__getitem__
+        def getparents(r):
+            return getrev(r)[5:6]
+        return getparents
+
     def takequicksample(self, headrevs, size):
         """takes a quick sample of size <size>
 
@@ -181,7 +187,7 b' class partialdiscovery(object):'
         if len(sample) >= size:
             return _limitsample(sample, size)
 
-        _updatesample(None, headrevs, sample, self._repo.changelog.parentrevs,
+        _updatesample(None, headrevs, sample, self._parentsgetter(),
                       quicksamplesize=size)
         return sample
 
@@ -191,10 +197,11 b' class partialdiscovery(object):'
             return list(revs)
         repo = self._repo
         sample = set(repo.revs('heads(%ld)', revs))
+        parentrevs = self._parentsgetter()
 
         # update from heads
         revsheads = sample.copy()
-        _updatesample(revs, revsheads, sample, repo.changelog.parentrevs)
+        _updatesample(revs, revsheads, sample, parentrevs)
 
         # update from roots
         revsroots = set(repo.revs('roots(%ld)', revs))
@@ -209,7 +216,6 b' class partialdiscovery(object):'
         # this by keeping a persistent cache of children across invocations.
         children = {}
 
-        parentrevs = repo.changelog.parentrevs
         for rev in repo.changelog.revs(start=min(revsroots)):
             # Always ensure revision has an entry so we don't need to worry
             # about missing keys.