Okay, @Falco 's solution worked, but I had to go about it in a round about way.
- I forked the original repo
- Cloned it locally on my dev machine
- Created my branch from the desired commit
git branch foo ec64ae8
- Pushed that branch to my github
git push -u origin foo
- Updated my app.yml with
git clone -b foo git@github.com:my-repo.git
Seems like there should be an easier way, but this gets the job done. Thanks for the direction @Falco !