dirstate-cext: properly invalidate mtime and data in `set_untracked` This was forgotten about in the initial implementation and was revealed while adding the `dirstate-v2` variant of `test-issue660.t`. Neither the existing Python implementation nor the upcoming Rust implementation suffer from this bug since they respectively have `None` and `Option<T>` to represent the lack of information. Differential Revision: https://phab.mercurial-scm.org/D12414