revlog: unify flag processing when loading index The new code use a simple declaration to do centralised processing. This is clearer, shorter and less error prone. This will be especially useful as we plan to add a fourth format: changelog-v2. Differential Revision: https://phab.mercurial-scm.org/D10622

revlog: unify checks for supported flag The new code use a simple declaration to do centralised checking. This is clearer, shorter and less error prone. This will be especially useful as we plan to add a fourth format: changelog-v2. Differential Revision: https://phab.mercurial-scm.org/D10621

side-data: drop the associated config and requirements This is no longer and extensions of revlog v1, but a part of revlog v2. We no longer needs independent config and requirement management for that. Differential Revision: https://phab.mercurial-scm.org/D10620

revlog: fix capitalisation of an error We don't start error message with capital letters. Differential Revision: https://phab.mercurial-scm.org/D10619

test: no longer directly use the sidedata config and requirements They are on their way out, so we simply use revlogv2 in these cases. Some test does not requires explicite request for the revlogv2 format since the upgrade to a side-data-capable format is impled by `exp-use-copies-side-data-changeset`. Differential Revision: https://phab.mercurial-scm.org/D10618

sidedata: add a 'side-data' repository feature and use it Most code don't really care how sidedata support is added, but it needs to know if it is present. To achieve this. we use the `repo.features` attributes with a new dedicated features". Having such centralised information is more robust and will help us to remove the sidedata requirements in a later changesets. Differential Revision: https://phab.mercurial-scm.org/D10617