GIT_EDITOR not working with macvim
You need to supply the -f option to vim to ensure that it doesn't background itself.
I had the same problem. I fixed it by setting mvim as the default editor for git by running the following from the command line:
git config --global core.editor "mvim -f"
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)