RFR: forge: use commit object in GitHubRepository.commitMetadata

[Erik Helin]

Hi all,

please review this patch that properly parses the JSON reply from the `/commits` endpoint on GitHub.

Testing:
- [ ] None, but this time I'm sure I got it right ��

Thanks,
Erik

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Commit messages:
 - forge: use commit object in GitHubRepository.commitMetadata

Changes: https://git.openjdk.java.net/skara/pull/661/files
 Webrev: https://webrevs.openjdk.java.net/skara/661/webrev.00
  Stats: 10 lines in 1 file changed: 1 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/661.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/661/head:pull/661

PR: https://git.openjdk.java.net/skara/pull/661