Fixes for 'filter_tag'

- Iterate over all tasks, not just the root elements.
- Consider all descendants of a tag, not just the
  immediate children.

GTG/core/tags.py

@@ -179,6 +179,15 @@ def get_ancestors(self) -> List['Tag']:
             ancestors.append(here)
         return ancestors
 
+
+    def get_matching_tags(self) -> List['Tag']:
+        """Return the tag with its descendants."""
+        matching = [self]
+        for c in self.children:
+            matching += c.get_matching_tags()
+        return matching
+
+
     def __hash__(self):
         return id(self)
 

GTG/core/tasks.py

@@ -1035,14 +1035,8 @@ def filter_tag(tag: Tag) -> List[Task]:
 
             output = []
 
-            for t in self.data:
-                tags = [_tag for _tag in t.tags]
-
-                # Include the tag's children
-                for _tag in t.tags:
-                    for child in _tag.children:
-                        tags.append(child)
-
+            for t in self.lookup.values():
+                tags = { matching_tag for own_tag in t.tags for matching_tag in own_tag.get_matching_tags() }
                 if tag in tags:
                     output.append(t)