qlog doesn't always show lines between tip children

[Vincent Ladeuil]

Disclaimer: this use case I have is rather involved but at least it reproduces the bug.

From bug #474807 (use a shared repo):
  bzr branch https://code.launchpad.net/~mysql/mysql-server/mysql-next-mr-bugfixing mr-bugfixing --no-tree
  bzr branch -r <email address hidden> sven --no-tree

  bzr qlog sven mr-bugfixing

shows no line between both tips whereas:

    bzr qlog mr-bugfixing sven

does.

Note that in the first case, clicking on the children link in the lower pane makes the missing line appear.

The missing line is not strictly speaking between tips but rather between one tip and an ancestor of the other,
yet not showing at least a dotted line is misleading.