add gitster theme

modules/prompt/functions/prompt_gitster_setup

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+../themes/gitster.zsh-theme

modules/prompt/themes/gitster.zsh-theme

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+#
+# Gitster theme
+# https://github.com/shashankmehta/dotfiles/blob/master/thesetup/zsh/.oh-my-zsh/custom/themes/gitster.zsh-theme
+#
+
+local ret_status="%(?:%{$fg_bold[green]%}➜ :%{$fg_bold[red]%}➜ %s)"
+
+function get_pwd(){
+  git_root=$PWD
+  while [[ $git_root != / && ! -e $git_root/.git ]]; do
+    git_root=$git_root:h
+  done
+  if [[ $git_root = / ]]; then
+    unset git_root
+    prompt_short_dir=%~
+  else
+    parent=${git_root%\/*}
+    prompt_short_dir=${PWD#$parent/}
+  fi
+  print $prompt_short_dir
+}
+
+PROMPT='$ret_status %{$fg[white]%}$(get_pwd) $(git_prompt_info)%{$reset_color%}%{$reset_color%} '
+
+ZSH_THEME_GIT_PROMPT_PREFIX="%{$fg[cyan]%}"
+ZSH_THEME_GIT_PROMPT_SUFFIX="%{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_DIRTY=" %{$fg[yellow]%}✗%{$reset_color%}"
+ZSH_THEME_GIT_PROMPT_CLEAN=" %{$fg[green]%}✓%{$reset_color%}"