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

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

It also means that *all* changes introduced by the PR are included instead of just the latest commit.  That seems right to me.  However, I've lost track of how people are thinking about stacked PRs now.

> The downside is that the merge target is hardcoded to main, which might not always be what we want. I still think this is an improvement over the status quo.

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

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