util: rename 're' to 'remod' Upcoming patches will introduce a binding called 're'.

bundle2-push: drop _pushbundle2extraparts All core user are now using the new way. We drop the old way.

bundle2-test: use the new way to extend push content The only core user of the old way were tests. We update them.

bundle2-push: introduce a list of part generating functions Instead of explicitly calling a few function to generate part in the bundle, we now have a list of all part generators. This should make it easier for extensions to adds new part in the bundle. This new way to extend the push deprecates the old `_pushbundle2extraparts` way.

bundle2-push: move changegroup push validation inside _pushb2ctx When bundle2 push includes more than just changesets, we may have no changegroup to push yet still have other data to push. So we now try to performs a bundle2 push in all cases. The check for changegroup inclusion is moved into the ``_pushb2ctx`` function in charge of creating the changegroup part. The bundle2 part is aborted if no actual payload part have been added to the bundle2.

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.