lib/git.zsh: Speed-up git_prompt_ahead()

Use the `git rev-list` plumbing instead of `git log`, which
allows us to do without grep.

Limit the history to one commit as well.

lib/git.zsh

@@ -57,7 +57,7 @@ git_remote_status() {
 
 # Checks if there are commits ahead from remote
 function git_prompt_ahead() {
-  if $(echo "$(git log origin/$(current_branch)..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
+  if [[ -n $(git rev-list -1 origin/$(current_branch)..HEAD 2>/dev/null) ]]; then
     echo "$ZSH_THEME_GIT_PROMPT_AHEAD"
   fi
 }