git integrations: Expand details in commit push notifications.

We now show a few new things:
(1) The number of commits pushed.
(2) Who authored the commits (just counts, not which specific ones, for brevity).

Add tests for case of multiple committers.

Part of #3968.

zerver/webhooks/github_webhook/view.py

@@ -180,6 +180,7 @@ def get_push_tags_body(payload):
 def get_push_commits_body(payload):
     # type: (Dict[str, Any]) -> Text
     commits_data = [{
+        'name': commit['committer']['username'],
         'sha': commit['id'],
         'url': commit['url'],
         'message': commit['message']