topic: add tests to demonstrate topic confuses the branchhead checking logic
While topics are in play, we store the branchheads (which has a topic)
in "branchname:topicname" format. After digging into it I found that
even in the case when we should have branch heads for "bname:tname"
we get heads for "bname".
The tests output reflect the confusion in branch head checking logic.
Next patch will be fixing the problem.