test-obsolete: sort the output of debugobsolete The set of relevant markers is currently unordered. Therefore the markers will be added in arbitrary order. We sort the list of markers beforehand to ensure stable output for testing.

test-obsolete: change a marker so it is relevant to the exchanged set We are going to only exchange markers relevant to the exchanged changesets. So we need to change this marker to use a known changeset as a successor instead of a precursor.

push: use bundle2 to push obsmarkers when possible

exchange: add a `buildobsmarkerpart` function We'll have to build an obsmarker part for both push and pull. So we build a function to factor out the common part.

obsolete: add a `commonversion` function This function returns the highest common version between the locally known formats and a list of remotely known formats. This is going to be useful to know what format should be used for exchange.

bundle2: add a `obsmarkersversion` function to extract supported version Right next to the function that encodes the supported versions in capabilities we add a function that decodes the versions out of capabilities. This is going to be useful to know what formats can be used for exchange.

bundle2: advertise the obsmarker part in bundle2 capabilities