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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

The new step essentially does what @llvm-beanz suggested in #70946. I'm not sure why this is so much faster than having the `changed-files` action fetch the history itself as they both do the same thing, but the results are definitely clear. This should avoid the issues with being unable to run on PRs with merge commits in #71131.

I still don't believe this accounts for the case where the author merges `main` back into their branch to update it and `main` has modified files that the author also touches with code formatting errors. I believe the action would currently pick them up even though they're not in the commit in question as they will show up between the start revision and the end revision due to the merge. That is a separate issue though and can be addressed in a follow up patch.

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