qlog: crash using --show-trees and multiple branches

[Gary van der Merwe]

bzr: ERROR: bzrlib.errors.NoSuchRevision: CHKInventoryRepository('file:///home/garyvdm/bug670284test/trunk/.bzr/repository/') has no revision current:/home/garyvdm/bug670284test/trunk

Traceback (most recent call last):
  File "/home/garyvdm/qbzr/trunk/lib/trace.py", line 320, in reports_exception_decorate
    return f(*args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/log.py", line 250, in load
    self.no_graph, gz_cls)
  File "/home/garyvdm/qbzr/trunk/lib/logwidget.py", line 101, in load
    self.log_model.load(*args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/logmodel.py", line 161, in load
    graph_viz.load()
  File "/home/garyvdm/qbzr/trunk/lib/logmodel.py", line 99, in load
    super(WithWorkingTreeGraphVizLoader, self).load()
  File "/home/garyvdm/qbzr/trunk/lib/loggraphviz.py", line 1172, in load
    super(WithWorkingTreeGraphVizLoader, self).load()
  File "/home/garyvdm/qbzr/trunk/lib/loggraphviz.py", line 207, in load
    head_revids, graph_parents = self.load_graph_parents()
  File "/home/garyvdm/qbzr/trunk/lib/loggraphviz.py", line 348, in load_graph_parents
    head_revs = self.load_revisions(head_revids)
  File "/home/garyvdm/qbzr/trunk/lib/logmodel.py", line 86, in load_revisions
    return load_revisions(revids, self.get_repo_revids)
  File "/home/garyvdm/qbzr/trunk/lib/lazycachedrevloader.py", line 97, in load_revisions
    for rev in repo.get_revisions(batch_revids):
  File "/usr/lib/python2.6/dist-packages/bzrlib/decorators.py", line 140, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/repository.py", line 1901, in get_revisions
    return self._get_revisions(revision_ids)
  File "/usr/lib/python2.6/dist-packages/bzrlib/decorators.py", line 140, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/repository.py", line 1909, in _get_revisions
    raise errors.NoSuchRevision(self, revid)
NoSuchRevision: CHKInventoryRepository('file:///home/garyvdm/bug670284test/trunk/.bzr/repository/') has no revision current:/home/garyvdm/bug670284test/trunk