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

[technicalpickles]

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

❯

[tstannard]

@perryqh can you take a look at this? TIA!

[tstannard]

cc @jackboberg

We (modularity team) would be happy to pair on this with you too!

[perryqh]

@tstannard is this a high priority? I'm definitely planning on fixing this and am happy to involve your team!