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)