dirstate-v2: Change the on-disk format when the requirement is enabled For now, the format is the same except with an additional marker at the start. This marker is redundant: for existing repositories it is `.hg/requires` that determines which format to use. For new repositories, it is the new `format.exp-dirstate-v2` config. There is no upgrade or downgrade so far. Most of the changes are about plumbing a boolean through layers of APIs to indicate which format should be used. Differential Revision: https://phab.mercurial-scm.org/D10719

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/// Added at the start of `.hg/dirstate` when the "v2" format is used.

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/// Acts like a "magic number". This is a sanity check, not strictly necessary

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/// since `.hg/requires` already governs which format should be used.

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pub const V2_FORMAT_MARKER: &[u8; 12] = b"dirstate-v2\n";