Undo: Writefile: fix 'id changed' detected when resorting IDs.

The ListBase next/prev pointers will change everytime you add or rename
an ID, also for 'neighbors' data-blocks in the list, causing unnecessary
'changed' detection.

This info is not needed in blendfile anyway, so just NULLify it.

source/blender/blenloader/intern/writefile.c

@@ -4141,6 +4141,11 @@ static bool write_file_handle(Main *mainvar,
         memcpy(id_buffer, id, idtype_struct_size);
 
         ((ID *)id_buffer)->tag = 0;
+        /* Those listbase data change everytime we add/remove an ID, and also often when renaming
+         * one (due to re-sorting). This avoids generating a lot of false 'is changed' detections
+         * between undo steps. */
+        ((ID *)id_buffer)->prev = NULL;
+        ((ID *)id_buffer)->next = NULL;
 
         switch ((ID_Type)GS(id->name)) {
           case ID_WM: