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

copies: move file input processsing early If we are to store the same kind of data outside of extra, we need to explicitly prepare them before that. On the long run, other storage (eg: sidedata) might use a different encoding to store this information, since the constraint from extra does not apply to it. Differential Revision: https://phab.mercurial-scm.org/D6937

copies: prepare changelog for more copies storage mode We want to store copies information in sidedata, the logic will be quite similar to the one used for extra, but sightly different (not in extra, no hash impact). We start with small refactoring to make our work easier and the coming changesets cleaners. Differential Revision: https://phab.mercurial-scm.org/D6936

vfs: give all vfs an options attribute by default Multiple code path assume vfs have an options attribute, including the vfs module itself. So for consistency we explicitly add one to all vfs. This will prevent various crash in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D6935