Fix infinite recursion

Force the Results to update its internal __dupes list whenever at least one group has re-prioritized and changed its dupes/ref.

core/app.py

@@ -770,6 +770,8 @@ class DupeGuru(Broadcaster):
         for group in self.results.groups:
             if group.prioritize(key_func=sort_key):
                 count += 1
+        if count:
+            self.results.refresh_required = True
         self._results_changed()
         msg = tr("{} duplicate groups were changed by the re-prioritization.").format(
             count

core/results.py

@@ -52,6 +52,7 @@ class Results(Markable):
         self.app = app
         self.problems = []  # (dupe, error_msg)
         self.is_modified = False
+        self.refresh_required = False
 
     def _did_mark(self, dupe):
         self.__marked_size += dupe.size
@@ -94,7 +95,9 @@ class Results(Markable):
 
     # ---Private
     def __get_dupe_list(self):
+        if self.__dupes is None or self.refresh_required:
             self.__dupes = flatten(group.dupes for group in self.groups)
+            self.refresh_required = False
             if None in self.__dupes:
                 # This is debug logging to try to figure out #44
                 logging.warning(