Non zsh/pcre version.

git-rprompt.zsh

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

autoload -Uz VCS_INFO_get_data_git; VCS_INFO_get_data_git 2> /dev/null

setopt prompt_subst

function rprompt-git-current-branch {
        local name st color gitdir action
        if [[ "$PWD" =~ '/\.git(/.*)?$' ]]; then
                return
        fi
        name=`git rev-parse --abbrev-ref=loose HEAD 2> /dev/null`
        if [[ -z $name ]]; then
                return
        fi

        gitdir=`git rev-parse --git-dir 2> /dev/null`
        action=`VCS_INFO_git_getaction "$gitdir"` && action="($action)"

        st=`git status 2> /dev/null`
	if [[ -n `echo "$st" | grep "^nothing to"` ]]; then
                color=%F{green}
	elif [[ -n `echo "$st" | grep "^no changes added"` ]]; then
                color=%F{yellow}
	elif [[ -n `echo "$st" | grep "^# Changes to be committed"` ]]; then
                color=%B%F{red}
        else
                color=%F{red}
        fi

        echo "$color$name$action%f%b "
}

RPROMPT='[`rprompt-git-current-branch`%~]'

詳しくはこちら。
http://www.yuuan.net/blog/index.php?itemid=856