Just provide the commit feed url as a query parameter, like so:
http://github-rss.heroku.com/?feed=https://github.com/username/project/commits/master.atomIf it is a private repo, you'll need to escape the '?' and '&' in the feed url with '%3f' and '%26' respectively, like so:
http://github-rss.heroku.com/?feed=https://github.com/username/project/commits/master.atom%3flogin=my_login%26token=1adefed234
Source is available on github.