dirstate-v2: freeze the on-disk format
It seems the format as reached a good balance. With a core of new capabilities
that motivated it initially and enough new feature and room for future
improvement to be a clear progress we can set a milestone for.
Having the format frozen will help the feature to get real life testing, outside
of the test suite.
The feature itself stay experimental but the config gains a new name to avoid
people enable non-frozen version by default.
If too many bugs are reported during the RC we might move the format back to
experimental and drop its support in future version (in favor of a new one)
Differential Revision: https://phab.mercurial-scm.org/D11709
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