checkheads: upgrade the obsolescence postprocessing logic (issue4354) The previous logic had many shortcoming (eg: looking at the head only, not handling prune, etc...), the new logic use a more robust approach: For each head, we check if after the push all changesets exclusive to this heads will be obsolete. If they are, the branch considered be "replaced". To check if a changeset will be obsolete, we simply checks: * the changeset phase * the existence of a marker relevant to the "pushed set" that affects the changesets.. This fixes two major issues of the previous algorithm: * branch partially rewritten (eg: head but not root) are no longer detected as replaced, * Prune are now properly handled. (This implementation was introduction in the evolve extension, version 6.0.0.) This new algorithm has an extended number of tests, a basic one is provided in this patch. The others will be introduced in their own changeset for clarity. In addition, we stop trying to process heads unknown locally, we do not have enough data to take an informed decision so we should stop pretending we do. This reflect a test that is now update.