obsolete: do not exchange extinct changesets Extinct changesets are excluded from all exchange operations. This is a silent exclusion because the user should not need to be aware of them. There is no reason to strongly enforce this exclusion except implementation simplicity. User should be able to explicitly request an extinct changeset in the future.

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.

localpeer: return only visible heads and branchmap Now that we have localpeer, we can apply filtering on heads and branchmap the same way it's done for wireprotocol peer.

util, posix: eliminate encodinglower and encodingupper 2ebe3d0ce91d claims this was needed "to avoid cyclic dependency", but there is no cyclic dependency. windows.py already imports encoding, posix.py can import it too, so we can simply use encoding.upper in windows.py and in posix.py. (this is a partial backout of 2ebe3d0ce91d)