Allow Zuul to process circular dependencies between changes. Gating of
circular dependencies must be explicitly enabled on a per tenant or
project basis.
In case Zuul detects a dependency cycle it will make sure that every
change also include all other changes that are part of the cycle. However
each change will still be a normal item in the queue with its own jobs.
When it comes to reporting, all items in the cycle are treated as one
unit that determines the success/failure of those changes.
Changes with cross-repo circular dependencies are required to share the
same change queue.
Depends-On: https://review.opendev.org/#/c/643309/
Change-Id: Ic121b2d8d057a7dc4448ae70045853347f265c6c
5161347efd