dirstate-v2: Add a new experimental `exp-dirstate-v2` repository requirement This requirement is added to `.hg/requires` when creating a new repository if Rust extensions are enabled and the `format.exp-dirstate-v2` config is set. Nothing yet changes based on this requirement, but its mere presence affects some tests (for example if they print `.hg/requires`). The next two changesets update tests’ expected outputs accordingly. There is no CI so far that enables this configuration. Differential Revision: https://phab.mercurial-scm.org/D10716