Fix for tryrebase after PR was merged (#91337)
rebasing certain merged prs results in the rebased branch pointing at the target branch b/c git believes the pr has already been included in the branch. Git does not replay the changes onto the target branch because the change is already in the target branch
This usually affects PRs with only 1 commit (more commits -> trymerge squashes them when merged -> git believes that the change is not in the target branch b/c the squashed commit is different from the individual changes).
It might also affect ghstack changes b/c behind the scenes the ghstack PRs are all contained within one commit on the orig branch, but I'm not sure about this.
helps w/ https://github.com/pytorch/test-infra/issues/836
looks like https://github.com/clee2000/random-testing/pull/44#issuecomment-1363439534
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91337
Approved by: https://github.com/ZainRizvi
