Description of bug

Diffing breaks in reviewstack when only one PR in a stack is resubmitted

Steps to Reproduce

1. Create a stack with "PR 1" and "PR 2" with any changes
2. Rebase PR 1 onto a new base (updated main perhaps)
3. sl pr submit on PR 1
4. View PR 2 in review stack, observe that the changes from main are included (even though PR 1/2 don't touch those files).

Possible Solutions

1. When any PR in a stack is updated, the whole stack should be resubmitted automatically (otherwise it leads to incorrect diff view)
2. When showing the changes of PR 2 in ReviewStack, it should not be diffed against the most recent version of PR1, but rather the version it was submitted with

Fix 2 seems preferred given it doesn't require resubmitting the whole stack to change one piece, not sure on technical limitations.