Allow for cherry-pick and revert "dry runs"
As part of gitlab-com/gl-infra/delivery#1009 (closed), teamDelivery needs to be able to know if a cherry-pick will apply cleanly to a target branch, but without actually applying the change. This will allow us to inform developers that a security fix requires a manual backport to a previous stable branch, or if we can simply cherry-pick their merge request targeting the main branch.
While we have no need (yet) for a revert dry-run, we can add that functionality at the same time since the APIs are nearly identical both in gitlab-rails and in Gitaly.