Use two-dot instead of three-dot diffs

[samskiter]

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This was raised on the old community-maintained issue tracker here: isaacs/github#750 (comment)

Additionally AzDO has a similar request out: https://developercommunity.visualstudio.com/t/pull-request-diff-show-actual-diff-instead-of-comm/1065332

As mentioned in that comment - having the wrong diff shown can lead to production outages

I notice this page explaining what github does:

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-comparing-branches-in-pull-requests#three-dot-and-two-dot-git-diff-comparisons

Since the three-dot comparison compares with the merge base, it is focusing on "what a pull request introduces."

That's not true at all - the NET change after the pull request is merged is the two-dot diff. To me it seems that it's cheaper / easier to implement 3-dot diffs as one only needs to recompute the diff if there is a push to the source branch, rather than the target branch.

Can we please at least have the option to show the two-dot diff?

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