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.