Rebase on partial merge shouldn't automatically dismiss approvals

[aleksei-averchenko-wise]

Automatic rebase on partial stack merge shouldn't automatically dismiss approvals, at least when the reviewable diff remains the same. It's a major usability gap, as illustrated by the following example:

1. I had a stack: main -> A ✅ -> B ✅ -> C ✅ - all approved.
2. I pressed the "Squash and merge" button in A.
3. Now I've got a stack: main -> A (merged) -> B ⌧ -> C ⌧ - my approvals on B and C are gone, and it might take hours for the code reviewers to re-stamp them.
4. (╯°□°）╯︵ ┻━┻