getbundle: add a ``cg`` boolean argument to control changegroup inclusion The ``getbundle`` function was initially design to return a changegroup bundle. However, bundle2 allows transmitting a wide range of data. Some bundle2 requests may not include a changegroup at all. Before this changeset, the client would request a changegroup for ``heads=[nullid]`` and receive an empty changegroup. We introduce an official boolean parameter, ``cg``, that can be set to false to disable changegroup generation on getbundle. A new bundle2 capability is introduced to let the client know.