Report on files being validated against

Identified at https://github.com/checkstyle/eclipse-cs/pull/380#issuecomment-1310507743 ,

It was not clear why we were getting random issues and then it became clear patch-filters prevented a Checkstyle run in some cases, which is its purpose.

It would be helpful if patch-filters reported something minor like "No files identified as changed" or "Identified X files as changed" so it is clear when allows Checkstyle to run and not run.

I don't think it would have helped this case, but it will make what it is doing clearer.

Start by locating the patch-filters logic that identifies changed files and decides whether to run Checkstyle; no source file or test is named in the issue. Trace that decision path and its existing output. Done means the filter reports when no files were found or how many files were identified, making the Checkstyle decision understandable.

Written by the indexing model from the issue text.