github webhooks: Fix message for unsupported team payloads.

The main thing fixed here is that we weren't turning
on our keys into a list.  And then I refined the message
a bit more, including sorting the keys.

I also avoid the unnecessary "else".

zerver/webhooks/github/tests.py

@@ -452,4 +452,8 @@ A temporary team so that I can get some webhook fixtures!
         stack_info = m.call_args[1]["stack_info"]
 
         self.assertIn("content_type: application/json", msg)
+        self.assertIn(
+            "summary: The 'team/edited (changes: bogus_key1/bogus_key2)' event isn't currently supported by the GitHub webhook",
+            msg,
+        )
         self.assertTrue(stack_info)

zerver/webhooks/github/view.py

@@ -280,8 +280,9 @@ def get_team_body(helper: Helper) -> str:
     if "privacy" in changes:
         new_visibility = payload["team"]["privacy"]
         return f"Team visibility changed to `{new_visibility}`"
-    else:
-        raise UnexpectedWebhookEventType("GitHub", f"Team Edited: {changes.keys()}")
+
+    missing_keys = "/".join(sorted(list(changes.keys())))
+    raise UnexpectedWebhookEventType("GitHub", f"team/edited (changes: {missing_keys})")
 
 def get_release_body(helper: Helper) -> str:
     payload = helper.payload