Backup subtree cache when adding new remote

.utils/add-subtree.sh

@@ -36,6 +36,10 @@ else
 fi
 
 if [ "${prevremotedir}" != "" ]; then
+    if [ -e "${path}/.subtree-cache" ]; then
+        # Backup subtree cache
+        cp -rT "${path}/.subtree-cache" "${prevremotedir}/.subtree-cache"
+    fi
     rm -r "${path}"
     mv -T "${prevremotedir}" "${path}"
 fi