Force fetching items from database if it has unforeseen commits

SpineDBWorker._do_fetch_more() now checks if database has commits from
unknown source before bailing out after iterating the db mapping.
This fixes cases where
1. fetch parent has fetched some data already, e.g. the Base alternative
2. an external source adds more data to the database, e.g. Importer brings
   new alternatives
3. fetch parent never fetches the new items since db worker would bail out
   after iterating the mapping

Re #2353

spinetoolbox/spine_db_worker.py

@@ -134,7 +134,7 @@ def fetch_more(self, parent):
             self._do_fetch_more(parent)
 
     def _do_fetch_more(self, parent):
-        if self._iterate_mapping(parent):
+        if self._iterate_mapping(parent) and not self._db_map.has_external_commits():
             # Something fetched from mapping
             return
         item_type = parent.fetch_item_type