bundle2: advertise bundle2 caps in server capabilities
We can now retrieve them from the server during push. The capabilities are
encoded the same way as in `replycaps` part (with an extra layer of urlquoting
to escape separators).
.. 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