revlog: introduce a `sidedata` method The method give access to extra information related to the revision. Such data will not be part of the hash be strongly related to the revision. Having them stored at the revlog level helps the storage consistency story and simplify various things. Example of data we could store there: - copy tracing related informations - graph structure related information (useful for discovery) - unresolved conflict data The full implementation will be introduced gradually in the coming changesets. Differential Revision: https://phab.mercurial-scm.org/D6808