topic: invalidate the topic cache when branchcache is invalidated

The branchmap is cleared in the superclass function, but I didn't see any sign
that this is ever cleared.

This fixes an issue in TortoiseHg where stale topic labels appear in certain
cases, such as if a commit has a topic and is stripped, and an unrelated commit
is pulled in from a remote repo.  Additionally, it fixes an issue with the topic
display in TortoiseHg and cmdserver when using `hg rollback`.  In the new test,
the result of the command that applies `topic2` without this fix was:

    6 d (topic2)
    5 c (topic2)
    4 b (topic1)
    0 a ()

The tie in was that rev 4 used to be `topic1` before the rollback.  Also note
that without the fix, the final log command printed the following, even though
it showed no signs of `topic1`, and nothing should have changed:

    active topic 'topic1' grew its first changeset
    (see 'hg help topics' for more information)
8 jobs for topic/stable/invalidate-topic-cache in 16 minutes and 45 seconds (queued for 2 seconds)