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
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..