qlog: Crash on right click on rev after commit, before refresh

[Gary van der Merwe]

steps to reproduce:
(~/qbzr/0.19 has uncommited changes)
pwd = ~/qbzr/0.19
bzr qlog ../trunk/ . &
commit
switch to qlog, right click on any rev

bzr: ERROR: exceptions.KeyError: 'key <email address hidden> not in nodes'

Traceback (most recent call last):
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/logwidget.py", line 560, in show_context_menu
    filter_rev_ancestor(self.context_menu_tag)
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/logwidget.py", line 550, in filter_rev_ancestor
    top_revid, is_ancestor)
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/logwidget.py", line 596, in filter_rev_ancestor
    self.graphprovider.known_graph.heads((branch_tip, rev)))
  File "_known_graph_pyx.pyx", line 444, in bzrlib._known_graph_pyx.KnownGraph.heads
KeyError: 'key <email address hidden> not in nodes'

The revid in the traceback is the newly committed rev.