Use low level command to detect branches behind

Using git log is subject to formatting in .gitconfig, which can be
overridden and will break this function. Relying on rev-list is much
more stable.

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/$(current_branch)..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
+  if [[ -n "$(command git rev-list origin/$(current_branch)..HEAD 2> /dev/null)" ]]; then
     echo "$ZSH_THEME_GIT_PROMPT_AHEAD"
   fi
 }