Repos Take on Their Multi-armed Form: When updating comment/PR body, use payload URI.

> We knew the world would not be the same. A few people laughed, a few people cried, most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says, "Now I am become Death, the destroyer of worlds."
> I suppose we all thought that, one way or another.
> — [Robert Oppenheimer](http://en.wikiquote.org/wiki/Robert_Oppenheimer)

We were calculating the URL ourselves, which meant chef-repo stuff would
affect the mercury repo, because we were calculating the URL with
respect to the mercury repo. Since the Github API gives us the URL as
part of the payload, we now just use that.

Fixes #11 .