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