bundle2: don't advertise stream bundle2 capability when feature disabled
The server.uncompressed config option can be used to disable streaming
clones. While the top-level capability to advertise streaming clone
support isn't advertised when this option is set, we were still sending
the bundle2-level capabilities advertising support for stream parts.
It makes sense to not advertise that support when streaming clones
are globally disabled.
If the structure of the new code seems a bit odd, it is because we'll
have to further tweak the behavior in commit(s) ahead.
Differential Revision: https://phab.mercurial-scm.org/D1929
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors