RFR: forge: make HostedCommit extend Commit

[Erik Helin]

Hi all,

please review this small refactoring that makes `HostedCommit` extend `Commit`.

Thanks,
Erik

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Commit messages:
 - forge: make HostedCommit extend Commit

Changes: https://git.openjdk.java.net/skara/pull/899/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=899&range=00
  Stats: 53 lines in 1 file changed: 0 ins; 49 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/899.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/899/head:pull/899

PR: https://git.openjdk.java.net/skara/pull/899