One commit, one plan: a commit off the trunk — a pull request's head, a branch built by
hand, a rebuild or replay of one — is for checking. The build seat reads from its clone
which branches hold the commit, and the controller records and never registers a build
whose commit is not on the branch the module follows (the repository's default for a
new one), so nothing off the trunk can be sent.
A pull request's check now carries its change plan, computed by the planner: what a
merge would build in which order, what each machine would receive, and what is not an
ordinary send — the bus step, a module waiting for a person, a provider's consumers.
