Question: how to have ls-files use the .gitignore in the index

I have a pre-commit hook that checks the index for files that match the ignores / excludes in the repository. But sometimes this will run on a repository where not all the folders are up-to-date with the tip, and will then test against the working tree versions of these files and not those in the index.

from pre-commit:

	if git ls-files -i --exclude-standard | grep .
	then
	    exit 1
	fi 

Is there an option to have ls-files use the excludes in the index without checking out the whole file tree?

Thanks,