GIT_BRANCH=
GIT_BG_PID=0

trap 'git_callback' USR1

get_git_branch () {
    git rev-parse --abbrev-ref HEAD 2> /dev/null > "/tmp/zsh_git_branch.$$"
    # git rev-parse --verify REBASE_HEAD 2> /dev/null && echo 1 > "/tmp/zsh_git_rebase.$$"
    kill -s USR1 $$ > /dev/null 2> /dev/null
}

git_branch_bg () {
    if [[ $GIT_BG_PID != 0 ]]; then
        kill -s HUP $GIT_BG_PID > /dev/null 2>&1
    fi
    if [ -f "/tmp/zsh_git_branch.$$" ]; then
        rm "/tmp/zsh_git_branch.$$"
    fi
    get_git_branch &!
    GIT_BG_PID=$!
}

git_callback () {
    GIT_BRANCH=$(cat "/tmp/zsh_git_branch.$$")
    rm "/tmp/zsh_git_branch.$$"
    update_right_prompt
    GIT_BG_PID=0
}

autoload -U add-zsh-hook
# add-zsh-hook chpwd git_branch_bg
add-zsh-hook precmd git_branch_bg
git_branch_bg