[Update] Save git diff only when local changes exist (#3350)

update.sh

@@ -370,8 +370,10 @@ DIFF_FILE=${DIFF_DIRECTORY}/diff_before_update_$(date +"%Y-%m-%d-%H-%M-%S")
 echo -e "\e[32mSaving diff to ${DIFF_FILE}...\e[0m"
 mkdir -p ${DIFF_DIRECTORY}
 mv diff_before_update* ${DIFF_DIRECTORY}/ 2> /dev/null
-git diff --stat > ${DIFF_FILE}
-git diff >> ${DIFF_FILE}
+if ! git diff-index --quiet HEAD; then
+  git diff --stat > ${DIFF_FILE}
+  git diff >> ${DIFF_FILE}
+fi
 
 echo -e "\e[32mPrefetching images...\e[0m"
 prefetch_images