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