Long I have been procrastinating over this. Whenever I had to quit out of the Vim while doing git-rebase interactively I would be going over to a new terminal window, and perform
git rebase --abort and then close the other window too.
Today, I learned if you ever want to quit Vim either while committing or rebasing, just do
:cq. This quits Vim with an error code, and Git will abort the action.