When you try to git stash and get some error like "error: unable to unlink old" with permission denied with it, it is at is says permission problem, but not on that file, but directory where that file is. So go to that folder where the problematic file is (usually it will be default folder) and give that folder permissions to write for user and group.
sudo chmod -R ug+w .with that, this should be solved and you can stash and pull git.