RFR: 8366399: Allow custom base reference for update_copyright_year.sh

[Erik Joelsson]

On Thu, 28 Aug 2025 22:50:27 GMT, Chen Liang <liach at openjdk.org> wrote:

> Currently, `update_copyright_year.sh` hardcodes `master` to be the base reference. While this works on mainline and update release repos, this does not work for projects like Valhalla or Leyden, whose PRs target lworld/premain branches. We can add a new flag to allow customized base reference to adapt to these use cases.

Marked as reviewed by erikj (Reviewer).

-------------

PR Review: https://git.openjdk.org/jdk/pull/26998#pullrequestreview-3168731439