debugobsolete: add a way to record parent information We add a ``--record-parents`` flag to debugobsolete. This can be used to record parent information in the marker when the precursors are known locally. This will be useful to test the "relevant markers" computation.

obsstore: add relevantmarkers method We add a ``relevantmarkers`` method to fetch all markers that seem relevant to a set of nodes. See function documentation about how this set is computed. This will let us exchange only the markers that seem "relevant" to the set of changesets related to a push or a pull. The approach used to define "relevant" has been successfully tested in evolve for 6 months.

obsstore: keep track of children information We use the new `parents` field to build a dictionary of markers that touch children of a node. This will be used to link prune markers to a set of exchanged nodes.

push: check if local and remote support evolution during discovery We can now directly prevent any evolution-related operation from happening by using an empty set for outgoing markers. So we detect if no transfers should occur and use an empty set in this case.

obsstore: drop outdated comment This comment was associated with a now-defunct line.

cvsps: add two more tiebreakers in cscmp test-convert-cvs.t has been a little flaky for a while now. Add an extra tiebreaker in cscmp so that all the cases in the test will sort reliably. Without this patch, test-convert-cvs.t failed after 346 runs. With this patch, I stopped trying to get it to fail after 615 runs. While not conclusive, that makes me pretty optimistic that this is a working fix.