Recently I saw this field in my heroku application settings.
Link your app to a GitHub repository to see commit diffs in the activity log.
If you want to see your code in Github you must add Github as a remote repository
git remote add origin https://git.heroku.com/repo.git
git remote set-url --add origin https://github.com/username/repo.git
Every time you push your commits they will push to both heroku and github
git push origin
Now when you click on the commit links, they will take you to github.