Currently, when I have to do it, here are the steps that I follow:
Is there a more straightforward way to do that?
Checkout the previous state of the file and amend the commit.
git checkout HEAD^ -- file.txt
git commit --amend
If you don't need to amend the commit log message the last command can be
git commit --amend -C HEAD
which doesn't even open your editor to edit the commit log, it just amends it.