[llvm] [Github] Use three dot diff for darker in code format action (PR #74704)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> 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. :laughing: 

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.

https://github.com/llvm/llvm-project/pull/74704