dart fix: report when --compare-to-golden has unchanged files
dart fix: report when --compare-to-golden has unchanged files

I was pretty confused about what was being compared until this change
was able to highlight that no edits were made.

Bug: https://github.com/dart-lang/sdk/issues/47678
Change-Id: I1d37070437658063d4b84f2fb5f4a33923bf5d92
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221840
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk