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.