Add a File to a Previous Commit with Interactive Rebase

We're going to pick a previous commit and enter an interactive rebase with:

git rebase -i HEAD~2

and change the word pick to edit on the commit where we want to add a file.

Then during the interactive rebase, we can add the file, and amend the commit we stopped on with:

git commit --amend --no-edit

and then once we're happy, continue the rebase with:

git rebase --continue