CONTRIBUTING

rust: Align DirstateEntry internals with Python/C DirstateItem

This propagate to this Rust struct the similar change that was made recently
to the Python classe and C struct. Namely, instead of storing a four-valued
`state` field we now store seven (bit-packed) booleans that give lower-level
information.

Additionally, the marker values -1 and -2 for mtime and size should not
be used internally anymore. They are replaced by some combinations of booleans

For now, all uses of of `DirstateEntry` still use the compatibility APIs
with `state` and marker values. Later the Rust API for DirstateMap
will be increasingly updated to the new style.

Also change the expected result of the test_non_normal_other_parent_entries
unit test. Only a `DirstateEntry` with `size == -2 && mtime != -1` is
affected, but this case never occurs outside of unit tests.
`size == -2` was the marker value for "from other parent" entries,
where no meaningful mtime is stored.

Differential Revision: https://phab.mercurial-scm.org/D11484

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