webhooks/clubhouse: Account for bug in GitHub branch payloads.

This change was prompted by a possible bug on Clubhouse's end. In
general, if a branch is added, it also prompts a workflow state
transition in its primary story.

However, our webhook error logs show an erroneous payload where
the same branch is added to another story, but there is no
workflow state transition. Also, both the stories are grouped in
the same payload, which confused/broke our code. Ideally, this
shouldn't happen and is most likely a bug on Clubhouse's end.
In most cases, changes included in Clubhouse payloads never
pertain to more than one parent entity (stories) simultaneously,
and we usually operate under the assumption that the changes
included therein are related to each other in terms of their
parent object (story or epic) and not a child object (the GitHub
branch).

zerver/webhooks/clubhouse/view.py

@@ -325,18 +325,16 @@ def get_reference_by_id(payload: Dict[str, Any], ref_id: int) -> Dict[str, Any]:
 
     return ref
 
-def get_story(payload: Dict[str, Any]) -> Dict[str, Any]:
-    story = {}  # type: Dict[str, Any]
-    for a in payload['actions']:
-        if a['entity_type'] == 'story':
-            story = a
-
-    return story
-
 def get_story_create_github_entity_body(payload: Dict[str, Any],
                                         entity: str) -> str:
     action = get_action_with_primary_id(payload)
-    story = get_story(payload)
+
+    story = {}  # type: Dict[str, Any]
+    for a in payload['actions']:
+        if (a['entity_type'] == 'story' and
+                a['changes'].get('workflow_state_id') is not None):
+            story = a
+
     new_state_id = story['changes']['workflow_state_id']['new']
     old_state_id = story['changes']['workflow_state_id']['old']
     new_state = get_reference_by_id(payload, new_state_id)['name']