RFR: vcs: lookup does not handle non-existing commits

[Erik Helin]

Hi all,

please review this patch that makes `ReadOnlyRepository.lookup` handle hashes that do not exist in the repository. I also added a unit test for this scenario.

Thanks,
Erik

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Commit messages:
 - vcs: lookup does not handle non-existing commits

Changes: https://git.openjdk.java.net/skara/pull/898/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=898&range=00
  Stats: 38 lines in 3 files changed: 38 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/898.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/898/head:pull/898

PR: https://git.openjdk.java.net/skara/pull/898