phases: fix bug where native phase computation wasn't called

I forgot to include this change as a previous diff and the native code to
compute the phases was never called. The AttributeError was silently caught and
the pure implementation was used instead.

mercurial/revlog.py

@@ -726,7 +726,7 b' class revlog(object):'
             return self._headrevs()
 
     def computephases(self, roots):
-        return self.index.computephases(roots)
+        return self.index.computephasesmapsets(roots)
 
     def _headrevs(self):
         count = len(self)