topic: fix the `topic(...)` revset to not select commits without a topic

The problem seems to be that the now removed contains check would select any revision indicated by a non-string argument. This led to topic(head()) returning a bunch of non-topic heads, though not as many as simply head().

I'm likely misunderstanding something, because there appears to be a similar contains check for branch() in core. But maybe that's just harder to see the issue, because branch names doesn't disappear like topics.