topic: add some initial support for using stack on named branch Stack is a useful command that can make sense in other context. The current support is hacky and awful, but we have something!

topic: explicitly pass topic as a keyword argument This clarify all callers before adding more logic related to bare branch in stack.

topic: makes split keep the topic Refactor split to extract the preparation of the working directory in a separate function. This helps topic wrap this function to save topic before update and restore it after preparing the working directory just before the split.