I have a similar problem, and it is very specific to Windows 7 and RStudio. I can connect successfully with an Ubuntu machine, and I can get Git by itself to pull from and push to my Github repository using the Windows machine. I have tried to troubleshoot with "Happy Git and Github for the useR", to no avail. The problem is on a Windows 7 machine that was working successfully for my with RStudio and Github before I started trying to change from https to ssh (then back again), and before updating Git, R, and RStudio. When trying to push or pull using RStudio, I currently get
>>> C:/Program Files/Git/mingw64/bin/git.exe pull
error: cannot spawn sh: No such file or directory
error: cannot spawn sh: No such file or directory
error: cannot spawn rpostback-askpass: No such file or directory
fatal: could not read Username for 'https://github.com': No such file or directory