rust: Remove EntryState::Unknown...
rust: Remove EntryState::Unknown
This enum variant represented the `state == '?'` case, which was used
to represent the absence of a dirstate entry/item (and therefore of that
entry’s state).
Now that previous refactors have removed this use in the Python/Rust
FFI APIs, the remaining uses can be removed by replacing `EntryState`
by `Option<EntryState>` where appropriate, using `None` to represent
the absence of an entry.
Differential Revision:
https://phab.mercurial-scm.org/D11465
