sidedata: make sure we don't use the flag if there are not sidedata Ensuring this at this level seems safer than relying on caller doing the right thing. G: changed mercurial/revlog.py Differential Revision: https://phab.mercurial-scm.org/D6895

revlog: add the appropriate flag is sidedata are passed to `addrevision` If we need to store sidedata, we need the flag to be set and the associated processing to be called. Differential Revision: https://phab.mercurial-scm.org/D6894

sidedata: register the flag processors if the repository allows for it Differential Revision: https://phab.mercurial-scm.org/D6893

sidedata: add a function for _processflagsraw usage Entry with side data cannot directly validate their rawtext Differential Revision: https://phab.mercurial-scm.org/D6892

sidedata: add a function to write sidedata into a raw text Differential Revision: https://phab.mercurial-scm.org/D6891

sidedata: add a function to read sidedata from revlog raw text This implement the "reading" part of a `sidedata` flag processor. Differential Revision: https://phab.mercurial-scm.org/D6890

sidedata: add a new module with basic documentation For now the storage strategy is very simple. We can augment it in the future if needed. Code to actually support what is described will be introduced in later changesets. Differential Revision: https://phab.mercurial-scm.org/D6889

sidedata: add a new revision flag constant for side data Small step first. Differential Revision: https://phab.mercurial-scm.org/D6888

upgrade: detect the side-data format variants Note that for now we cannot upgrade/downgrade to it. Differential Revision: https://phab.mercurial-scm.org/D6887

sidedata: introduce a new requirement to protect the feature The feature will be usable only within repository supported by version of Mercurial with the support. Since the support is not fully implemented yet, the requirement has a temporary, experimental name. Differential Revision: https://phab.mercurial-scm.org/D6886