Install MacVim and the command line tool, mvim.
Then in your ~/.gitconfig add this, thusly (replacing the path to mvim with the correct one):
[merge]
tool = mvim
[mergetool “mvim”]
cmd=/Users/tjweir/bin/mvim -d -g $LOCAL $MERGED $REMOTE
keepbackup=false
Then you can “git mergetool” all you’d like.