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.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html