topic: setup 'topic' value for working ctx The way amend work in core change in e8a7c1a0565a. The intermediate commit disappear and we can no longer rely on it to carry the topic value to amend. To fix this, we ensure the current topic value can be retrieved through the `workingctx.extra()` value. Since there is no way to carry a "missing" key information in a way that survives and "dict.update" call. We also has to introduce hacking behavior about a special empty value for topic.

tests: abstract version number for test-topic-stack.t Using explicite version number always bring trouble, this is why we have unique description that can select changesets.