Homebrew update fail: “Please, commit your changes or stash them before you can merge”

I was able to resolve the issue myself.

What tipped me off is running “git status” did not show that file.

Instead of using the common solution:

cd $(brew --repository)
git reset --hard FETCH_HEAD

I had to do:

cd [directory of the file in question]
git reset --hard FETCH_HEAD

That resolved the problem.