RFR: 1563: The method 'CheckRun#updateMergeReadyComment' shouldn't update the comment if the comment has not changed

[Zhao Song]

Do not update comment if the comment already exists.

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Commit messages:
 - do not update comment if the comment already exists

Changes: https://git.openjdk.org/skara/pull/1386/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1386&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1563
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1386.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1386/head:pull/1386

PR: https://git.openjdk.org/skara/pull/1386