List current remotes:
git remote -v
Add Heroku product & staging remotes:
Using Heroku-CLI:
heroku git:remote -a staging-app-name -r staging
heroku git:remote -a production-app-name -r production
Set staging as default for "heroku" command:
git config heroku.remote staging
If you need to rename your previously added "heroku" remote:
git remote rename heroku staging