ci: ESLint checks more files than it should

How can we otherwise help?

When a PR branch is not perfectly up-to-date with master, the ESLint workflow checks both the files changed in the PR (which it should) and the files changed on master (which it shouldn't in my opinion). In the case of my PR #8715 it causes the job to fail (logs), because it can't find a file added to master in my PR branch.

Here is the job log, showing the files checked by ESLint:

and here is the list of files actually touched by my PR:

This issue can be fixed by changing the workflow step slightly:

-FILES=$(git diff --diff-filter=ACM --name-only "$BASE" "$HEAD" \
+FILES=$(git diff --diff-filter=ACM --name-only "$BASE"..."$HEAD" \
  | grep -E '\.(js|json|md)$' || true)