RFR: 1215: Clean label not updating correctly

[Zhao Song]

Originally, If the user mistakenly creates a Backport PR with the wrong hash and then later corrects it, the clean status is never correctly re-evaluated. 

Currently, If the user correct the hash, the bot will be forced to do the check again and could correctly add clean label.

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Commit messages:
 - Make clean label updating correctly

Changes: https://git.openjdk.org/skara/pull/1388/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1388&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1215
  Stats: 128 lines in 2 files changed: 126 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1388.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1388/head:pull/1388

PR: https://git.openjdk.org/skara/pull/1388