The new GitHub post-commit hook handler had some issues in production.

First, when adding the hook, GitHub would try to send a "ping" event,

which we did not support. This resulted in GitHub saying that the hook

wasn't functioning properly (though it wouldn't disable the hook).
Second, and more importantly, the hook signature verification failed.

The X-Hub-Signature header contained the prefix "sha1=", which the

handler wasn't expecting. It now looks for this, erroring out if it

doesn't find it.

New unit test changes pass. Haven't tried this in production yet.