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

update: clarify calculateupdate() call sites by specifying argument names merge.calculateupdate() takes a lot of parameters and I get confused all the time which is which. See also b14fdf1fb615 (update: clarify update() call sites by specifying argument names, 2017-02-09). Differential Revision: https://phab.mercurial-scm.org/D6883