To syntax highlight git's commit messages, you need to:
  1. Copy syntax/gitcommit.vim to vim's syntax directory:
     $ mkdir -p $HOME/.vim/syntax
     $ cp syntax/gitcommit.vim $HOME/.vim/syntax
  2. Auto-detect the editing of git commit files:
     $ cat >>$HOME/.vimrc <<'EOF'
     autocmd BufNewFile,BufRead COMMIT_EDITMSG set filetype=gitcommit
     EOF