Reviews are interpreted like comments and can contain any number of
commands, with the difference that APPROVED and REQUEST_CHANGES are
interpreted as (respectively) r+ and r- prefixes.
* p0 cancel existing stagings in order to be staged as soon as
possible
* p0 PRs should be picked over split batches
* p0 bypass PR-level CI and review requirements
* p0 can be set on any of a batch's PR, matched PRs will be staged
alongside even if their priority is the default