treediscovery: fix regression when run against older repos (
issue2793)
I ran the entire test suite with "known" and "getbundle" disabled in
localrepository. This generated failures because the old findoutgoing
had always queried remote's heads explicitly and thus always got them
back in the returned heads. treediscovery.findcommonincoming now
correctly returns remote's heads in all cases.
Also adds a dedicated test for running treediscovery against a
pre-getbundle HTTP server.