Fast-forward master in the same run as seeing the passing tests

[brson]

Currently there's a two-step process for merging a successful PR: see that the tests passed and post the status to github; see that the branch is ready for merge and fast-forward. This is two invocations of bors' cron job, which for Rust is 5 minutes apart. This is costing us at least an hour of down time every day.

[reiddraper]

+1. I might be able to grab this in the next couple of weeks.