revlog: add a `sidedata` parameters to addrevision If we want to eventually store sidedata we need to be able to pass them along. Differential Revision: https://phab.mercurial-scm.org/D6814

flagprocessors: have the read transform function return side data (API) This makes it possible for flag processors to -read- flag data. Differential Revision: https://phab.mercurial-scm.org/D6813

flagprocessors: return flagdata in the main processing function This function input and return are becoming stranger and stranger bnut I don't have a good plan to make is saner without problematic code duplication, so it will be this way to now. Differential Revision: https://phab.mercurial-scm.org/D6812

flagprocessors: return sidedata map in `_processflagsread` Right now, flag processors does not return sidedata, by they will. So, we prepare the caller to receive it. Differential Revision: https://phab.mercurial-scm.org/D6811

revlog: use the new sidedata map return in the sidedata method So far things, seems logical. Differential Revision: https://phab.mercurial-scm.org/D6810

revlog: return sidedata map from `_revisiondata` Nothing extra any side data yet. However, it will happens in the future. So we better prepare the callers of the `_revisiondata` to deal with it. Differential Revision: https://phab.mercurial-scm.org/D6809

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