[llvm] [Github] Fetch all commits in PR for code formatting checks (PR #69766)

[Tobias Hieta via llvm-commits]

tru wrote:

Oh in my previous testing sparse checkout was really really slow. But that sounds good then! LGTM feel free to merge, but do it when you can monitor the actions tab and see if the jobs don't fail in an unexpected way. 

https://github.com/llvm/llvm-project/pull/69766