qlog: unabe to show graph for merge of 2 roots

[Alexander Belchenko]

Bazaar allows to merge two separate branches that have no common ancestry at all, with command:

bzr merge -r0..-1 path/to/another/branch

I often use this trick, and BzrTools project too. Recently Aaron Bentley merged full history of 'heads' plugin into bzrtools, see revno.637 in lp:bzrtools branch.

When I'm trying to expand this merge by clicking on + sign the graph appears broken at all (see attached screenshot) and in the console there is traceback:

Traceback (most recent call last):
  File "C:\work\Bazaar\plugins-repo\qbzr\lib\log.py", line 545, in changesList_mouseReleaseEvent
    self.changesModel.colapse_expand_rev(index, not twisty_state.toBool())
  File "C:\work\Bazaar\plugins-repo\qbzr\lib\logmodel.py", line 642, in colapse_expand_rev
    self.compute_lines()
  File "C:\work\Bazaar\plugins-repo\qbzr\lib\logmodel.py", line 399, in compute_lines
    parent_msri = self.revno_msri[parent_revno]
KeyError: (0,)