Integrated: vcs: add ReadOnlyRepository.contains for Hash

[Erik Helin]

Hi all,

please review this small patch that adds an overload `ReadOnlyRepository.contains` for just a `Hash` that checks if the repository does contain the given hash.

Testing:
- [x] Added a new unit test

Thanks,
Erik

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Commit messages:
 - vcs: add ReadOnlyRepository.exists

Changes: https://git.openjdk.java.net/skara/pull/925/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=925&range=00
  Stats: 28 lines in 5 files changed: 24 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/925.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/925/head:pull/925

PR: https://git.openjdk.java.net/skara/pull/925