I was able to clone a copy of this repo over HTTPS authenticated. I\'ve made some commits and want to push back out to the GitHub server. Using Cygwin on Windows 7 x64.
The other answers that suggest switching to SSH sort of miss the point. HTTPS is supported, but you must log in with you GITHUB password, not your SSH passphrase (which was what was giving me the same exact error).
I was having the same problem, but making sure to use my actual GitHub password at the terminal password prompt fixed the solution with no alteration to the config, or resorting to SSH.
The reason it is important to note this, is many public institutions (such as my school) will block SSH, but allow HTTPS (which is the only reason I started cloning over HTTPS in the first place).
Hope that helps anyone else having the same issue...