On GitHub we cannot reliably update all information that's needed for
doing a canmerge check using events. Namely completely missing events
on branch protection changes and ambiguous status events that might
match several changes due to its data model to have statuses on the
commit instead the pr. This was no problem in the past since this
information was only used during the enqueue phase which is directly
after the event preprocessing phase.
However with circular dependencies we re-do the can merge check just
before merging again and need to act on recent data. Therefore add an
allow_refresh flag that makes it possible to refresh the volatile
parts of the data we don't get events for. This is only used on GitHub
for now as the other drivers are either correctly updating their
states using events or didn't yet optimize to not do api calls within
the main loop yet (pagure).