Bug(feature?) on git-diff-files

[NoName Req <nonamereq29@xxxxxxxxx>]

Summary
   `git -C (folder path) diff-files --name-only` output is not correct

Steps to Reproduce
1. Apply change to a clean git repository. Go to another folder (not
within this repository) and apply the command
   `git -C repo-path diff-files --name-only`
The above command lists dirty files in that repository.
2. Now undo your changes on the repository
3. Issue the same command. It still lists the file.

More Information

`git -C repo-path diff`  command doesn't seem to have this problem.

System Information

Arch Linux 5.0.7-arch1-1-ARCH
git version 2.21.0