Add update statistics

After the upgrade is interesting to know what's new.

tools/upgrade.sh

 printf '\033[0;34m%s\033[0m\n' "Upgrading Oh My Zsh"
 cd "$ZSH"
-if git pull --rebase origin master
+if git pull --rebase --stat origin master
 then
   printf '\033[0;32m%s\033[0m\n' '         __                                     __   '
   printf '\033[0;32m%s\033[0m\n' '  ____  / /_     ____ ___  __  __   ____  _____/ /_  '