If you have GitHub for Windows (installed, it should come with your shortcut.
It is a shortcut to:
C:\Users\Username\AppData\Local\GitHub\GitHub.appref-ms --open-shell
More recent versions of G4W (see answer below) could have it at:
%LOCALAPPDATA%\Apps\2.0\...\...\
C:\Users\Username\AppData\Local\Apps\2.0\GitHub\GitHub.appref-ms --open-shell
If that shell complains about the absence of git, launch “G4W” itself, which will extract git.
See “Where is git.exe located?”.
As mentioned below, to restore the shortcut, after having run the first command, execute in the Git shell:
github --reinstall-shortcuts