Create PushPayloads with Git

Review Request #10022 — Created June 14, 2018 and submitted — Latest diff uploaded

Information

rb-gateway
master
e800746...

Reviewers

The Git implementation of parsing a push event is no longer a panic!
It now successfully handles the output of a post-receive hook. This
will allow us to install a Git hook that can hit Review Board to close
review requests mentioned in commits pushed to a Git repository.

Ran go fmt ./....
Ran go test ./....