feature parity: don't fail when untracked git files aren't in codeowners

I just noticed this while testing. I had an untracked file that the ruby version is fine with, but the rust version wants to add

Ruby:

```
❯ bin/codeownership validate --diff
❯
```

Rust:

```
❯ bin/codeowners-rs generate-and-validate

Some files are missing ownership
- script/count-cursorignore.rb


❯
```