Move staging cancellation to the batch, remove its (complicated)
handling from the PRs.
This loses some precision in the cancellation message, but that could
likely be recovered (in part) by adding more precise checks &
diagnostic extractions in the compute.
Simplifies the `ready_prs` query a bit and allows it to be converted
to an ORM search, by moving the priority check outside. This also
allows the caller to not need to post-process the records list
anywhere near the previous state of affairs.
`ready_prs` now returns *either* the "alone" batches, or the non-alone
batches, rather than mixing both into a single sequence. This requires
correctly applying the search filters to not retrieve priority of
batches in error or targeting other branches.
Staging readiness is a batch-level concerns, and many of the markers
are already there though a few need to be aggregated from the PRs. As
such, staging has no reason to be performed in terms of PRs anymore,
it should be performed via batches directly.
There is a bit of a mess in order not to completely fuck up when
retargeting PRs (implicitly via freeze wizard, or explicitely) as for
now we're moving PRs between batches in order to keep the
batches *mostly* target-bound.
Some of the side-effects in managing the coherence of the targeting
and moving PRs between batches is... not great. This might need to be
revisited and cleaned up with those scenarios better considered.
- `merge_date` should be common to an entire batch, so move it there
- remove `Batch.active` which should probably have been removed when
batches were made persistent (can eventually re-add as a proxy for
`merge_date` being set maybe, but for now removing it seems a better
way to catch mistakes)
- update various sites to use `Batch.merge_date` instead of
`Batch.active`
This probably has latent bugs, and is only the start of the road to v2
(#789): PR batches are now created up-front (alongside the PR), with
PRs attached and detached as needed, hopefully such that things are
not broken (tests pass but...), this required a fair number of
ajustments to code not taking batches into account, or creating
batches on the fly.
`PullRequests.blocked` has also been updated to rely on the batch to
get its batch-mates, such that it can now be a stored field with the
right dependencies.
The next step is to better leverage this change:
- move cross-PR state up to the batch (e.g. skipchecks, priority, ...)
- add fw info to the batch, perform forward-ports batchwise in order
to avoid redundant batch-selection work, and allow altering batches
during fw (e.g. adding or removing PRs)
- use batches to select stagings
- maybe expose staging history of a batch?
