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
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