topic: take topic in account for all branch head computation This changeset introduce a "topicmap" that is tracking not just the head of all branches, but the heads of all branch+topic pair. Including the head of the part of the branch without any topic. In practice this means that BRANCHNAME now resolve to the tipmost part for the branch without topic and impact various other logic like head checking during push and default destination for update and merge (these aspect will need adjustment in later changesets). The on-the-fly-temporary-monkey-patching process is pretty horrible, but allow to move forward without waiting on having core patched. We use 'branch:topic' as the branchmap key, this is a small and easy hack that help use a lot for (future) support of heads discovery/checking and on disc cache. I'm not sure it is worthwhile to improve this until an implementation into core. Note that this changeset change the branchmap in all cases, including during exchange, see next changeset for improved behavior. We also currently have the on-disk cache disabled because the core branchmap is lacking phase information in its cache key. This will get done in a later changesets