changing-files: rework the way we store changed files in side-data We need to store new data so this is a good opportunity to rework this fully. 1) We directly store the list of affected file in the side data: * This avoid having to fetch and parse the `files` list in the revision in addition to the sidedata. Making the data more self sufficient. * This work around situation where that `files` field contains wrong information, and open the way to other bug fixing (eg: issue6219) * The format (fixed initial index, sorted files) allow for fast lookup of filename within the structure. * This unify the storage of affected files and copies sources and destination, limiting the number filename stored redundantly. * This prepare for the fact we should drop the `files` as soon as we do any change affecting the revision schema. * This rely on compression to avoid a significant increase of the changelog.d. More testing on this will be done before we freeze the final format. 2) We can store additional data: * The new "merged" field, * A future "salvaged" set recording files that might have been deleted but have were still present in the final result. Differential Revision: https://phab.mercurial-scm.org/D9090