`cargo rail unify` doesn't actually detect unused dependencies

If I run

```
cargo init --lib my-lib
cd my-lib
git add .
git commit -m "init"
cargo add eyre
```

then `cargo rail unify --check` exits with status 0, and `cargo rail unify` doesn't notice that `eyre` is unused, even though `cargo machete`, `cargo udeps`, and `cargo shear` can find it just fine.