Did you try inserting the username (and optionally password) in the clone URL?
git clone https://username@github.com/blah/blahblah.git
or if you accept the consequences of storing your password in plain view:
git clone https://username:password@github.com/blah/blahblah.git
See this thread with a lot of good info:
How to provide username and password when run “git clone git@remote.git”?
EDIT: My original answer was just a quick fix, without understanding the full history of the asker, and it also works unattended, but for best security practices its better to let Git store your password. See @phpguru’s answer: https://stackoverflow.com/a/29018371/257090 as well.