disable graphing
https://bugs.launchpad.net/bzr/+bug/541626

minimal testcase from
https://github.com/termie/git-bzr-ng/issues/38

=== modified file 'revision_store.py'
--- revision_store.py	2012-01-10 08:48:02 +0000
+++ revision_store.py	2012-11-03 22:06:31 +0000
@@ -170,7 +170,7 @@
         """
         self.repo = repo
         self._graph = None
-        self._use_known_graph = True
+        self._use_known_graph = False
         self._supports_chks = getattr(repo._format, 'supports_chks', False)
 
     def expects_rich_root(self):