The man page says
ENVIRONMENT AND CONFIGURATION VARIABLES The editor used to edit the commit log message will be chosen from the GIT_EDITOR environment variable, the core.editor configuration variable, the VISUAL environment variable, or the EDITOR environment variable (in that order).
So in order to make nano our default editor we can do
git config --global core.editor "nano"
--global means this will become a default setting for all of git (leaving it out would only affect the current git repo you are running the command from)