Better GitHub alerts

Rather than the erroneous "X pushed Y" (which really meant "commit Y by
X was pushed"), a slightly clearer message (plus a pseudo testing
harness)

Also, alerting on force-push
Will this render as `str(list())`? E.g. "['01243...', 'abcde...', '67890...']" because then I think I would prefer additional formatting. Also, what is the "head_commit" message?

Finally, can we extend this to have a separate message for merging PRs? Because otherwise "author" is more useful than "pusher", I think. Thoughts?