clfilter: split `revlog.headrevs` C call from python code

Make the pure python implementation of headrevs available to derived classes. It
is important because filtering logic applied by `revlog` derived class won't
have effect on `index`. We want to be able to bypass this C call to implement
our own.

mercurial/revlog.py

@@ -607,7 +607,9 b' class revlog(object):'
         try:
             return self.index.headrevs()
         except AttributeError:
-            pass
+            return self._headrevs()
+
+    def _headrevs(self):
         count = len(self)
         if not count:
             return [nullrev]