Deleting the source branch when merging a cross-repo pull request fails

Issue Summary

The 'delete source branch and retarget affected pull requests' checkbox is available on cross repo PRs, when it shouldn't be. Cross-repo PRs can not have their 'affected' pull requests retargetted since pull requests live in the target repository and these pull requests have already been created in the source repository and they cannot be moved into the target repositories.

Additionally, if a pull request has any dependent cross-repo PRs, then the source branch cannot be deleted. This is incorrect because the dependent cross-repo PRs should be able to be safely retargetted.

Steps to Reproduce

Described above

Expected Results

Should not be able to delete the source branch off cross-repo PRs.

Should be able to delete the source branch on non-cross-repo PRs that have dependent cross-repo PRs.

Actual Results

Can delete the source branch off cross-repo PRs but then retargetting fails.

Cannot delete the source branch when there are dependent cross-repo PRs

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available