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

[Chen Liang]

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.

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Commit messages:
 - 8366399: Allow custom base reference for update_copyright_year.sh

Changes: https://git.openjdk.org/jdk/pull/26998/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26998&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366399
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26998.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26998/head:pull/26998

PR: https://git.openjdk.org/jdk/pull/26998