This is what I was looking for. Thanks to notnoop for the lead I needed. I wanted to post back my solution in case it helps others.
git diff HEAD --name-only
Since I intended to do
git commit -s -F mesage.txt
with the files found in the first line.
My intent is to create a little system that totally ignores the index i.e. that I never need to do git add. (From what I understand, the index is useful when creating patches, which isn’t by no means the norm in my workflow.)