Show branch name only version.

git-rprompt.zsh

#
# Show branch name in Zsh's right prompt
#

function rprompt-git-current-branch {
    local name
    if [[ -n `echo "$PWD" 2> /dev/null | grep '/\.git(/.*)?$'` ]]; then
        return
    else
        name=`git rev-parse --abbrev-ref=loose HEAD 2> /dev/null`
        if [[ -z $name ]]; then
            return
        else
            echo "$DARK:$YELLOW$name$RED"
        fi
    fi
}

RED=$'%{\e[1;31m%}'
YELLOW=$'%{\e[0:33m%}'
DARK=$'%{\e[1:30m%}'

RPROMPT=$RED'[%~`get-git-current-branch`]%{${reset_color}%}'

詳しくはこちら。
http://www.yuuan.net/blog/index.php?itemid=866