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