RFR: 2159: CheckRun repeats search for backport commit on each evaluation

[Zhao Song]

As Erik said in the issue description, when skara bot is searching for the original commit for a backport port, it's very inefficient for GitLab because GitLab doesn't provide the good rest api as GitHub for the bot to search for a commit by hash. So Skara have no choice but go through all the repositories and it's very slow right now.

To fix it, we could store the repository name together with the commit hash in the comment, therefore the bot will only need to run the search once.

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Commit messages:
 - update
 - test

Changes: https://git.openjdk.org/skara/pull/1605/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1605&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2159
  Stats: 73 lines in 11 files changed: 40 ins; 4 del; 29 mod
  Patch: https://git.openjdk.org/skara/pull/1605.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1605/head:pull/1605

PR: https://git.openjdk.org/skara/pull/1605