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.