test-bundle2: add obsolescence information to be exchanged To introduce a bundle2 way to exchange obsolescence markers, we need to have some information available to exchange. Introduce markers relevant to changesets involved in the exchange. The new markers reference the changesets as successor nodes of clowny (nonexistent) hashes so that other than being exchanged they have no effect. We introduce them in two waves as push is expected to be smart about the number of markers it exchanges sooner than pull.