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
Added signature for changeset
181e52f2b62f