rhg: Add Repo::write_dirstate
This method is not used yet. It saves to disk any mutation that was done to
the `Repo`’s dirstate through `Repo::dirstate_map_mut`. It takes care of
dirstate-v1 v.s. dirstate-v2, dockets, data files, appending when possible,
etc.
Differential Revision: https://phab.mercurial-scm.org/D11839
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