Merged PRs should have a batch which should have a staging, this makes
the treatment uniform across the board and avoids funky data which is
hard to place or issues when reconstructing history.
Also create synthetic batches & stagings for older freezes (and bumps)
Initially wanted to skip this only for FW PRs, but after some thinking
I feel this info could still be valuable even for non-fw PRs which
were never merged in the first place.
Requires a few adjustments to not break *everything*: `batch.prs`
excludes closed PRs by default as most processes only expect to be
faced by a closed PR inside a batch, and we *especially* want to avoid
that before the batch is merged (as we'd risk staging a closed PR).
However since PRs don't get removed from batches anymore (and batches
don't get deleted when they have no PRs) we now may have a bunch of
batches whose PRs (usually a single one) are all closed, this has two
major side-effects:
- a new PR may get attached to an old batch full of closed PRs (as
batches are filtered out on being *merged*), which is weird
- the eventual list of batches gets polluted with a bunch of
irrelevant batches which are hard to filter out
The solution is to reintroduce an `active` field, as a stored compute
field based on the state of batch PRs. This way if all PRs of a batch
are closed it switches to inactive, and is automatically filtered out
by search which solves both issues.
This is definitely non-trivial, due to the structural changes and the
amounts of stuff to move around (e.g. lift from PR to batch), as well
as the reification of previously non-existent relations (batches,
batch history, ...) which sometimes uncovers inconsistencies in the
current state of the mergebot (some of which are the result of bugs,
the bug got fixed but the nonsense it generated was left untouched).
