dirstate-v2: Change the on-disk format when the requirement is enabled...
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
