refactor: helper: Simplify the topic indexing block in index_messages.

This extracts msg_topic and narrow_topics as variables and amends the
conditional accordingly.

Author: preetmishra

zulipterminal/helper.py

@@ -403,12 +403,14 @@ def index_messages(messages: List[Message],
                 (index['stream_msg_ids_by_stream_id'][msg['stream_id']]
                  .add(msg['id']))
 
-        if (msg['type'] == 'stream' and len(narrow) == 2
-                and narrow[1][1] == msg['subject']):
-            topics_in_stream = index['topic_msg_ids'][msg['stream_id']]
-            if not topics_in_stream.get(msg['subject']):
-                topics_in_stream[msg['subject']] = set()
-            topics_in_stream[msg['subject']].add(msg['id'])
+        if msg['type'] == 'stream' and len(narrow) == 2:
+            narrow_topic = narrow[1][1]
+            msg_topic = msg['subject']
+            if narrow_topic == msg_topic:
+                topics_in_stream = index['topic_msg_ids'][msg['stream_id']]
+                if not topics_in_stream.get(msg_topic):
+                    topics_in_stream[msg_topic] = set()
+                topics_in_stream[msg_topic].add(msg['id'])
 
     return index