[FIX] do submodule sync before submodule update

fixes "fatal: reference is not a tree" if remote is changed

update

@@ -19,6 +19,7 @@ if [ -d "site" ]; then
     echo "Updating t3kit ..."
     if [ -z "$(git status --porcelain)" ]; then
         git pull
+        git submodule sync
         git submodule update --init --recursive
         cd TYPO3.CMS
         composer install