I believe using --no-index
is what you're looking for:
git diff [<options>] --no-index [--] <path> <path>
as mentioned in the git manual:
This form is to compare the given two paths on the filesystem. You can omit the
--no-index
option when running the command in a working tree controlled by Git and at least one of the paths points outside the working tree, or when running the command outside a working tree controlled by Git.