RFR: 827: Copyright year differences can be regarded as a clean backport

[Zhao Song]

As many user requested, when determining whether a backport is clean, our bot should ignore copyright year difference.

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Commit messages:
 - SKARA-827

Changes: https://git.openjdk.org/skara/pull/1556/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1556&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-827
  Stats: 49 lines in 3 files changed: 45 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1556.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1556/head:pull/1556

PR: https://git.openjdk.org/skara/pull/1556