Provide a way to view the diff for a pull request branch

What steps will reproduce the problem?

1. Using GitHub or GitLab, create a pull request from my-pr-branch into the master branch
2. Commit some changes to my-pr-branch
3. Merge the master branch into my-pr-branch
4. Commit some more changes to my-pr-branch
5. Merge the master branch into my-pr-branch
6. Commit some more changes to my-pr-branch

What is the expected output? What do you see instead?

When you view the pull request on the GitHub/GitLab website, the diff shows only the changes from steps 2, 4, and 6. The changes from steps 3 and 5 are omitted, because they were merged from the base branch. They are not part of the pull request.

This diff can be seen locally by running the command git diff master...my-pr-branch (with three dots)

Expected: TortoiseGit should provide some way to show this diff.

Actual: As far as I can tell, TortoiseGit provides no way to view a PR diff, even though reviewing PRs is one of the most common activities in everyday work today.

What version of TortoiseGit and Git are you using? On what operating system?

2.8.0.0 on Windows 10

Please provide any additional information below.

(TortoiseGit does have an option for "Show log of ..." with three dots. But this opens a new log window, not a diff. I have no idea what this command is useful for.)