After using my git-todo a bit I quickly became annoyed by empty TODO files so I added a check to remove the file if it is empty after leaving vim.

function git-todo {
    OS=$(uname -s)
    local SED_OPT
    if [ "$OS" == "Darwin" ]; then
        SED_OPT="-E"
    fi
    if [ "$OS" == "Linux" ]; then
        SED_OPT="-r"
    fi
    local GIT_DIR=$(git rev-parse --git-dir)
    if ! (( $? )); then
        GIT_DIR=$(echo "$GIT_DIR" | sed $SED_OPT 's/\/?\.git$//')
        if [ -z "$GIT_DIR" ]; then
            GIT_DIR=.
        fi
        local TODO="$GIT_DIR/TODO.taskpaper"
        vim "$TODO"
        if [ ! -s "$TODO" ]; then
            rm -f "$TODO"
        fi
    fi
}

Also on GitHub, .bashrc.d/git.sh.