bundle2: call _pushbundle2extraparts a bit sooner This is the first step of a refactoring that will ease the inclusion of new part in the bundle2 push and includes more information (like phases) in this push We need to move the function a bit sooner to be able to group the generation of `b2x:check:heads` and `b2x:changegroup` part in an external function. We move it sooner to preserve parts creation order bundle2 tests rely on. At the ends of this refactoring the `_pushbundle2extraparts` will be replaced by another mechanism anyway.