discovery: port _postprocessobsolete() changes from evolve, add tests Let's make this function obsolescence-aware as well. Now we check that revisions are not obsolete first, and only then check that they are going to be common after exchange. The tests make sure that rewriting changesets doesn't confuse head computation. They rely on experimental.single-head-per-branch feature to show that remote correctly sees the expected result (i.e. either aborts the push or allows it to succeed). They are ported from evolve as well. Differential Revision: https://phab.mercurial-scm.org/D12098

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