replaced hardcoded origin/{branch-name} with @{upstream} which gets the upstream branch since git 1.7.0

lib/git.zsh

@@ -54,7 +54,7 @@ git_remote_status() {
 
 # Checks if there are commits ahead from remote
 function git_prompt_ahead() {
-  if $(echo "$(command git log origin/$(current_branch)..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
+  if $(echo "$(command git log @{upstream}..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
     echo "$ZSH_THEME_GIT_PROMPT_AHEAD"
   fi
 }