Improve "(see what's changed)" to show difference between

When a pull request is updated after an approval, then a "(see what's changed)" link appears.

This currently links to a diff between the commit of the approval and the latest commit of the pull request.

The pain I experience with this, is that if the new commit is a merge from master (e.g. an automerge from your CI tool), then the diff will show all the files that have changed on master, which the developer is not usually concerned about. It does not take into account the pull request destination, which already has these files.

Ideally, I would like this link to show how the files unique to the pull request have changed. I.e. limited to changes made directly on the branch, not including merge commits.

I have tried to workaround this by changing the destination of the diff to master, but this shows the whole pull request changes, not just the updates.

Example

1. Branch commit 1 - a.txt
2. Merge back into branch from master - z.txt
3. Branch commit 2 - b.txt

Expected for "see what's changed" between step 2 and step 1: "No changes"
Expected for "see what's changed" between step 3 and step 1 or step 2: "b.txt"