To avoid having to enter your username and password EVERY-SINGLE-TIME you push, follow these step-by-step instructions.
Go to https://c9.io/account/ssh and copy the key below "Connect to your private git repository". It's a very long string that starts with ssh-rsa and ends with your email.
Go to https://github.com/settings/keys and click New SSH key
.
Enter a title (a name like Cloud9 is fine), paste the ssh key that you copied in the previous step, and click Add SSH key
That's it! Next time you push anything, it should authenticate automatically, and you'll never again have to enter your credentials on C9.