sidedata: add a debugsidedata command The command provide basic capabilities to inspect revision side data. Differential Revision: https://phab.mercurial-scm.org/D6897

sidedata: test we can successfully write sidedata For now we just write them and rejoice on the lack of crashes. Differential Revision: https://phab.mercurial-scm.org/D6896

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