Add on betwen comment action and noun in comments messages from git integrations.

Improve English grammar in comment messages in git integrations
by adding 'on' between comment action and noun (PR, Issue, Commit).

zerver/views/webhooks/github.py

@@ -72,7 +72,7 @@ def github_object_commented_content(payload, type):
     # type: (Mapping[text_type, Any], text_type) -> text_type
     comment = payload['comment']
     issue = payload['issue']
-    action = u'[commented]({})'.format(comment['html_url'])
+    action = u'[commented]({}) on'.format(comment['html_url'])
 
     return get_pull_request_event_message(
         comment['user']['login'],