If I receive the mail notification that staging(-next) merge failed,
I either need to check `git log staging-next` or click the action run link
to find out where should I resolve the conflict.
To save time, let’s include the information about which step failed right in the comment.
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.
zowoq