The Mercurial SCM implementation now supports parsing the environment

created by a changegroup event (which occurs when the repository

receives a set of commits) and creating a PushPayload. This will allow

us to install a Mercurial hook that can hit Review Board to close review

requests mentioned in commits pushed to a Mercurial repository.
The Git implementation is currently a panic() since it cannot

be triggered by anything external. Git support is coming in a future

patch.

Ran go fmt ./....

Ran go test ./....