Could we also get exclusions from .gitignore?

From https://github.com/haskell/cabal/issues/11638

```
$ cabal haddock-project

$ fix-whitespace --check 2&> whitespace-violations.txt

$ grep "haddocks" .gitignore
haddocks
cabal-testsuite/**/haddocks
```

When a generated file or directory is already excluded with `.gitignore` could we opt-in to pick up those exclusions too?