When ever the mandatory message is not supplied to any Git command, by default Git Opens up the Vi Editor Screen as follows:
Vi has 2 modes, Edit mode and command mode and to type in a message in the Vi Screen, get in to Edit Mode by typing in ESC Key followed by i (Insert mode) and type in the required commit message. Once you have completed keying in required message, save the changes & get out of Vi editor by typing in ESC Key followed by :wq!