Get most recent commit id instead of relying on head_commit

[deniak]

The server logs show that the payload for a push event may not have a head_commit property.

[cgi:error] [pid 3506356:tid 3506356] [client xxx] AH01215: stderr from github-notify-ml/index.py:   File "github-notify-ml/index.py", line 70, in event_id
[cgi:error] [pid 3506356:tid 3506356] [client xxx] AH01215: stderr from github-notify-ml/index.py:     return payload["head_commit"]["id"]
[cgi:error] [pid 3506356:tid 3506356] [client xxx] AH01215: stderr from github-notify-ml/index.py:            ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
[cgi:error] [pid 3506356:tid 3506356] [client xxx] AH01215: stderr from github-notify-ml/index.py: TypeError: 'NoneType' object is not subscriptable

Looking at the documentation, it seems that property can indeed be null.

That PR updates the function to return the most recent commit id instead.