I believe the default is simply to open your default editor so just set $VISUAL (and $EDITOR, read the manual for your shell) in your environment. This has the added benefit (?) of using that editor for commands that open a visual editor such as crontab -e or even visudo (with some caveats of course).
If you want to use a different editor just for git you can set $GIT_EDITOR in your environment.
If you want to use a different editor just for git you can set $GIT_EDITOR in your environment.