topic: use fully qualified branch name during exchange
For the entire duration of exchange process, we now use branch//namespace/topic format for branchmap, head checking, etc. Sometimes we still need to disable this feature (e.g. when pushing to a repo that doesn't have topic extension enabled), but every other case should be handled using the new fqbn format. This applies both to internal API, such as having fqbn as branchmap keys and even making ctx.branch() return fqbn, and to UI, such as users seeing fqbn in hg branches output and messages from hg push. Things to note: now we're wrapping revbranchcache.branchinfo() during the extension setup, instead of doing it only in one place in _headssummary(). We're also using override_context_branch context manager that makes ctx.branch() return fqbn also in cases when we call the original _headssummary(). There are still places in the UI where branch names are not in fqbn format, because they don't take part in the exchange process.