Merge pull request #4189 from bmdhacks/master

agnoster: speed up git prompt

themes/agnoster.zsh-theme

@@ -78,7 +78,7 @@ prompt_git() {
 
   if $(git rev-parse --is-inside-work-tree >/dev/null 2>&1); then
     dirty=$(parse_git_dirty)
-    ref=$(git symbolic-ref HEAD 2> /dev/null) || ref="➦ $(git show-ref --head -s --abbrev |head -n1 2> /dev/null)"
+    ref=$(git symbolic-ref HEAD 2> /dev/null) || ref="➦ $(git rev-parse --short HEAD 2> /dev/null)"
     if [[ -n $dirty ]]; then
       prompt_segment yellow black
     else