Copyright year calculation with merge commits

Copyright year calculation currently takes the date of the merge commit, even if that particular file merged cleanly.

It causes a specific failure in spotlessCheck (I used Gradle plugin, but the issue is likely not related to Gradle): when branches being merged were changed in the previous year (e.g. 2025) and both had correct copyright end year, checked by Spotless at the time of their respective commits, and when the merge itself happens in the new year (e.g. 2026), Spotless insists that the 2025 copyright year from the clean merge is now incorrect and requires a manual change to 2026.

The expected outcome is that the clean, non-conflicting merge commit does not advance the copyright end year.
Merge commit that actively changes a file (to resolve conflicts or otherwise) should still advance the end year.

まず、著作権年の計算と、merge commit を対象とするテストまたは fixture を見つけます。issue ではファイルやエントリーポイントは指定されていません。異なる年に変更されたブランチとクリーンな merge を使って、説明されている Spotless のチェックを再現します。クリーンな merge では終了年が進まず、ファイルを変更する merge では引き続き終了年が進めば完了です。

索引モデルが issue の本文から書いたものです。