qlog: crash in refresh after using update to this revision.

[Gary van der Merwe]

bzr: ERROR: exceptions.KeyError: (1, 1)

Traceback (most recent call last):
  File "/home/garyvdm/qbzr/trunk/lib/util.py", line 870, in decorate
    run_in_loading_queue(f, *args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/util.py", line 882, in run_in_loading_queue
    f(*args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/uifactory.py", line 32, in decorate
    r = f(*args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/logwidget.py", line 109, in refresh
    self.load(*args, **kargs)
  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 201, in load
    self.compute_lines()
  File "/home/garyvdm/qbzr/trunk/lib/logmodel.py", line 211, in compute_lines
    computed = self.graph_viz.compute_viz(self.state)
  File "/home/garyvdm/qbzr/trunk/lib/loggraphviz.py", line 621, in compute_viz
    state.get_filtered_revisions()]
  File "/home/garyvdm/qbzr/trunk/lib/loggraphviz.py", line 1271, in get_filtered_revisions
    branch_line = self.graph_viz.branch_lines[branch_id]
KeyError: (1, 1)

This should be more resilient.