1cea247e6c tried to improve staging
checks to avoid staging PRs in the wrong state, however it had two
issues:
PR state
--------
The process would reset the PR's state to open, but unless the head
was being resync'd it wouldn't re-apply the statuses on the state,
leading to a PR with all-valid statuses, but a missing CI.
Message
-------
The message check didn't compose the PR message the same way PR
creation / update did (it did not trim the title and description
individually, only after concatenation), resulting in a
not-actually-existing divergence getting signaled in the case where
the PR title ends or the description starts with whitespace.
Expand relevant test, add a utility function to compose a PR message
and use it everywhere for coherence.
Also update the logging and reporting to show a diff of all the
updated items (hidden behind a `details` element).
23e1b93465