This is my custom git prompt for oh_my_zsh. It shows to current branch and the git dirty state (color). If a upstream exists, the difference is also displayed.

custom-git-prompt.sh

function git_prompt_info() {
  ref=$(git symbolic-ref HEAD 2> /dev/null) || return

  ZSH_THEME_GIT_PROMPT_PREFIX="("
  ZSH_THEME_GIT_PROMPT_SUFFIX=")$reset_color "
  ZSH_THEME_GIT_PROMPT_DIRTY="$fg[red]"
  ZSH_THEME_GIT_PROMPT_CLEAN=""

  local oh_my_git_string="";
  local has_upstream=false;

  oh_my_git_string+="$(parse_git_dirty)$ZSH_THEME_GIT_PROMPT_PREFIX$(git_current_branch)"

  local upstream=$(git rev-parse --symbolic-full-name --abbrev-ref @{upstream} 2> /dev/null)
  if [[ -n "${upstream}" && "${upstream}" != "@{upstream}" ]]; then
    local has_upstream=true;
  else
    local has_upstream=false;
  fi

  local current_commit_hash=$(git rev-parse HEAD 2> /dev/null)
  if [[ $has_upstream == true ]]; then
    local commits_diff="$(git log --pretty=oneline --topo-order --left-right ${current_commit_hash}...${upstream} 2> /dev/null)"
    local commits_ahead=$(grep -c "^<" <<< "$commits_diff");
    local commits_behind=$(grep -c "^>" <<< "$commits_diff");
  fi

  if [[ $has_upstream == true ]]; then
    if [[ ${commits_behind} -gt 0 ]]; then
      oh_my_git_string+="<";
    fi
    if [[ ${commits_ahead} -gt 0 ]]; then
      oh_my_git_string+=">";
    fi
    if [[ ${commits_behind} -eq 0 && ${commits_ahead} -eq 0 ]]; then
      oh_my_git_string+="=";
    fi
  fi

  oh_my_git_string+="$ZSH_THEME_GIT_PROMPT_SUFFIX"

  echo $oh_my_git_string
}