merge: refactor labels selection code The code is simplified to prepare the future introduction of a third labels for the merge base.

push: include phase push in the unified bundle2 push Phase push is now included in the same bundle2 push as changesets. We use multiple pushkey parts to transmit the information. Note that phase moves are still not part of the repository "transaction".

push: perform phases discovery before the push This will allow including phase information in the same bundle2 as the changesets.

push: make discovery extensible We need to gather all discovery before the unified bundle2 push. We use the same pattern as bundle2 parts generation.

push: rework the bundle2partsgenerators logic Instead of a single list of functions, we now have a list of names and a mapping of names to functions. This simplifies wrapping of steps from extensions. In the same move, declaration becomes decorator-based (syntax sugar, nom nom nom!).

push: move common heads computation into pushop Now that both options (push succeed or fall back) live in pushop, we can move the common heads computation there too. It is a very commonly accessed attribute so it makes a lot of sense to have it in pushop.

push: extract fallback heads computation into pushop Similar motivation to `futureheads`, we extract the computation into pushop to make it available early to all possibly interested parties.

push: extract future heads computation into pushop Bundle2 will allow pushing all different parts of the push in a single bundle. This mean that the discovery for each part needs to be done before trying to push. Currently we may have different behaviors for phases and obsolescence markers when the push of changesets fails. For example, information may still be exchanged for a part of the history where changesets are common but where phases mismatch. So the preparation of the push need to determine what information need to be pushed in both situations. And it needs a different set of heads for this. Therefore we are moving heads computation within pushop for easy access by all parties. We start with the simplest set of heads.