This is what I have in my ~/.bashrc
export GIT_EDITOR=\'/Applications/MacVim.app/Contents/MacOS/Vim -g \'
When I issue
git comm
This worked great for me.
git config --global core.editor 'mvim -f --nomru -c "au VimLeave * !open -a iTerm"'
If you are using Terminal.app
instead of iTerm.app
you can just switch it out.
Source: https://github.com/b4winckler/macvim/wiki/FAQ (though they are using the EDITOR env variable instead of the gitconfig)