debugobsolete: add a --rev argument This argument can be used to list markers relevant to a set of revisions. We add a test for this option and the relevant computation in the same move.

obsolete: rename `allmarkers` to `getmarkers` This function is going to gain a "nodes" argument to filter the markers to the ones relevant to <nodes> only.

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.