Automate the merging of `master` -> `staging-next` -> `staging`.
Our main development branch is `master`. Large rebuilds go to `staging`.
Periodically, `staging` is merged into `staging-next` for stabilization.
When considered sufficiently stable, `staging-next` is merged into
`master`.
As changes arrive on these branches, it is important that they're all
updated regularly with eachothers changes. This commit automates that
part.
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