qlog: Crash when searching for revision number

[Gary van der Merwe]

bzr: ERROR: exceptions.AttributeError: 'QLogGraphProvider' object has no attribute 'revno_index'

Traceback (most recent call last):
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/uifactory.py", line 31, in decorate
    r = f(*args, **kargs)
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/log.py", line 505, in update_search
    revid = self.log_list.graph_provider.revid_from_revno(revno)
  File "/home/garyvdm/.bazaar/plugins/qbzr/lib/loggraphprovider.py", line 1437, in revid_from_revno
    if revno not in self.revno_index:
AttributeError: 'QLogGraphProvider' object has no attribute 'revno_index'