[llvm] [ci] Diff against main when determining what files have changed for pre-commit CI (PR #67743)

[Louis Dionne via llvm-commits]

ldionne wrote:

> 
> Is there any way to parameterize that? Sorry, I'm not familiar with these scripts.

Right now it's possible to pass `MODIFIED_FILES` as an environment variable, however we can't customize the merge target. I don't think we have a concrete need for that right now, so let's punt. In the future, IMO the solution will be to move to Github actions instead and stop maintaining our custom triggering logic.

https://github.com/llvm/llvm-project/pull/67743