Use MacVim as your git diff tool (no external script)

Add this to your gitconfig file:

[diff]
    tool = mvimdiff
[difftool "mvimdiff"]
    cmd = mvim -f -d \"$LOCAL\" \"$REMOTE\"

You can now invoke it with git difftool $FILE to get local differences on a file in your gitrepo. Invoking it without any arguments runs through every modified file in your repo.