rhg: Add lazy/cached dirstate data file ID parsing on Repo
The `dirstate_parents`, `dirstate_data_file_uuid`, and `dirstate_map` members
of `Repo` can be access in any order and the `.hg/dirstate` file should only
be opened once.
Differential Revision: https://phab.mercurial-scm.org/D11838
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IncludeCategories:
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