[Github] Use three dot diff for darker in code format action

[boomanaiden154]

Using a two dot diff allows changes made in main after the merge base to show up in the formatting diff. Using a three dot diff fixes this and ensures that only changes made in the source branch (branch from the PR author) will get passed along to the formatter.

Without this, issues like #73873 occur.

[tru]

Ugh, the three dot vs two dots always confuses me and I never remember which is which. Thanks for fixing that.

[boomanaiden154]

Ugh, the three dot vs two dots always confuses me and I never remember which is which. Thanks for fixing that.

I will promptly forget all the details in a couple days once this is all fixed up. 😆

Still need to get this fixed on the clang-format side, but that will require a nightly git-clang-format right now as the patch enabling three dot diffs there just landed.