The changes introduced in a prerequisite branch are not in the scope of a code review. This is why they are not shown in the diff.
If you could change/add/remove the prerequisite branch, you would be changing the scope of the code review. By deleting a prerequisite branch, you could retroactively cause an approval of changes that the reviewer had never seen. By adding a merge proposal, you could remove changes that the reviewer might consider vital.
The changes introduced in a prerequisite branch are not in the scope of a code review. This is why they are not shown in the diff.
If you could change/add/remove the prerequisite branch, you would be changing the scope of the code review. By deleting a prerequisite branch, you could retroactively cause an approval of changes that the reviewer had never seen. By adding a merge proposal, you could remove changes that the reviewer might consider vital.