For modification of existing, unpushed commit messages we use following commands.
$git commit --amend
Or
$git commit --amend -m “new message”
Where
$git commit --amend
will open an editor where you can edit the message, and you can directly give the modified message to existing and unpushed commit messages.
Then for this question, you could use:
git add the_left_out_file
git commit --amend --no-edit