qlog: show tree action results in error dialog

Steps to reproduce (using latest version from trunk)
1) open qlog.
2) right click on single revision
3) select "Show Tree..." action.

Results in qbrowse being shown but an error dialog is also shown:

bzr: ERROR: exceptions.TypeError: '_known_graph_pyx._MergeSortNode' object is not iterable

Traceback (most recent call last):
  File "/home/craig/.bazaar/plugins/qbzr/lib/trace.py", line 286, in reports_exception_decorate
    return f(*args, **kargs)
  File "/home/craig/.bazaar/plugins/qbzr/lib/browse.py", line 147, in load
    self.set_revision(revision_id=self.revision_id, text=self.revision_spec)
  File "/home/craig/.bazaar/plugins/qbzr/lib/uifactory.py", line 31, in decorate
    r = f(*args, **kargs)
  File "/home/craig/.bazaar/plugins/qbzr/lib/browse.py", line 204, in set_revision
    self.revno_map = self.branch.get_revision_id_to_revno_map()
  File "/usr/lib/python2.6/dist-packages/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/branch.py", line 380, in get_revision_id_to_revno_map
    mapping = self._gen_revno_map()
  File "/usr/lib/python2.6/dist-packages/bzrlib/branch.py", line 400, in _gen_revno_map
    in self.iter_merge_sorted_revisions())
  File "/home/craig/.bazaar/plugins/historycache/__init__.py", line 175, in historycache_iter_msr_method
    br._revno_cache.update(br._merge_sorted_revisions_cache)
  File "/home/craig/.bazaar/plugins/historycache/revnocache.py", line 149, in update
    for rev_id, depth, revno, eom in revision_tuple_list:
TypeError: '_known_graph_pyx._MergeSortNode' object is not iterable

bzr 2.0.0 on python 2.6.2 (linux2)
arguments: ['/usr/bin/bzr', 'qlog']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_ZA.UTF-8'
plugins:
  bzrtools /usr/lib/python2.6/dist-packages/bzrlib/plugins/bzrtools [2.0.0]
  email /home/craig/.bazaar/plugins/email [unknown]
  explorer /home/craig/.bazaar/plugins/explorer [0.8.2]
  extmerge /home/craig/.bazaar/plugins/extmerge [unknown]
  gtk /home/craig/.bazaar/plugins/gtk [0.98.0dev1]
  historycache /home/craig/.bazaar/plugins/historycache [unknown]