Revlogv2 implies sidedata.
Right now sidedata is not really used in production, and Revlogv2 will be used
for the first production-ready version of sidedata support.
This whole part seems like a mistake during a merge. We should drop it.
if revlogv2 <=> sidedata, we should replace the associate format variant with this new one on adding a "duplicated" one.
revlogv1 should move to a constant for clarity/cleaness. Can you do that as preparatory changesets?