RFR: pr: return HostedCommit from CheckRun.backportedFrom

[Erik Helin]

Hi all,

please review this small refactoring that makes `CheckRun.backportedFrom` return the `HostedCommit` instead of just a `Hash`.

Thanks,
Erik

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Commit messages:
 - pr: return Commit from CheckRun.backportedFrom

Changes: https://git.openjdk.java.net/skara/pull/950/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=950&range=00
  Stats: 20 lines in 1 file changed: 8 ins; 7 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/950.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/950/head:pull/950

PR: https://git.openjdk.java.net/skara/pull/950