push: use `stepsdone` to control changegroup push through bundle10 or bundle20 We use the newly introduced `pushop.stepsdone` attribute to inform older methods than changegroup have already been pushed using a newer method.

push: add a ``pushop.stepsdone`` attribute This attribute will record what steps were performed during the bundle2 push. This will control whenever the old way push must be performed or skipped. This will ultimately be used by changegroup, phases, obsmarkers, bookmarks and any other kind of data ones may want to exchange even when bundle2 support is missing.

bundle2: add a ``bundle20.nbparts`` property This property can be used to know how much parts have been added to the bundle2. This will be useful to check if any part have been generated for a push.

bundle2-push: extract changegroup logic in its own function We extract the creation of changegroup related parts into its own function. This precludes the inclusion of more diverse data during the bundle2 push. We use a closure to carry the logic that need to be perform when processing the server reply.