[llvm] [Github] Fetch through merge base in code formatting action (PR #72020)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Thanks for the review! I'm going to land this and keep a close eye on it over the weekend to make sure there aren't any significant regressions before usage picks up on Monday.

https://github.com/llvm/llvm-project/pull/72020