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
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