repo: move visibleheads and visiblebranchmap logic in discovery They were previously inside the mercurial.phases module, but obsolete logic will need them to exclude `extinct` changesets from pull and push. The proper and planned way to implement such filtering is still to apply a changelog level filtering. But we are far to late in the cycle to implement and push such a critical piece of code (changelog filtering). With Matt Mackall approval I'm extending this quick and dirty mechanism for obsolete purpose. Changelog level filtering should come during the next release cycle.