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

Since we moved to Github PRs, the workflow has changed a bit and folks
often merge `main` back into their PR branch. This is fine, except the
previous way of determining modified files for pre-commit CI would use
the content modified just in the latest commit, whatever it is. This
means that in case someone merged main back into their PR branch, we'd
think that the files in the merge commit were modified by the PR, and
we'd spuriously trigger a CI run. This should fix this issue.

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.