review the working branches structure

There are six working branches: mesa-uprev{-tmp,,-wip}{-testonly}. And there could be two types of merge requests in virglrenderer: the drafted (when expectations update is present) and the clean one (when the uprev is clean and doesn't need to update any expectation).

This set of things could lead to confusion that should be addressed. For example, a draft merge request must have removed this flag before merging, which may collide with a previous existing merge request of the other kind. This situation may produce misunderstandings when developers like to act over those merge requests.

We don't want to create separate daily merge requests. This case will produce too much noise in the proposed merge requests of virglrenderer. That's the main reason behind checking if it already exists and appending a note.