Install the latest version of MacVim. Yes, MacVim. And yes, the latest.
If you don’t use the MacVim GUI, it is recommended to use the Vim binary that is inside the MacVim.app package (MacVim.app/Contents/MacOS/Vim). To ensure it works correctly copy the mvim script from the MacVim download to your local binary folder (for example /usr/local/bin/mvim) and then symlink it:
alias vim="/Applications/MacVim.app/Contents/MacOS/Vim" git config --global core.editor /Applications/MacVim.app/Contents/MacOS/Vim
It requires Vim 7.3.885 or later with Lua support (“+lua”).