topic-revset: strictly read string Making a clear distinction between string and symbol passed to topic is important when it comes to empty topic. Filtering an empty topic name is harmless. Searching a non existent revision is not.

test: search for empty topic as a string The reduce the ambiguity in the test to help with the next change.

topic: handle ambiguous arguments to topic() revset These arguments can be interpreted as either string or a revset. The decision is made based on existence of topic with such a name. This matches the behavior of branch() revset. The code needs to know all topics that ever existed in the repo, because some commands report "disappearance" of topics after certain operations, using this revset (e.g. via stack.stack or repo.revs).

topic: handle revsets passed to topic() revset This handles cases when the argument can't be interpreted as a string at all.

topic: handle string argument to topic() revset earlier

topic: prepare to handle non-string arguments to topic() revset

topic: return result early if there are no arguments to topic() revset