RFR: vcs: add support for cherry picking

[Erik Helin]

Hi all,

please review this patch that adds support for "cherry picking" commits to `Repository`.

Testing:
- [x] Added two new unit tests

Thanks,
Erik

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Commit messages:
 - vcs: add support for cherry picking

Changes: https://git.openjdk.java.net/skara/pull/921/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=921&range=00
  Stats: 100 lines in 4 files changed: 100 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/921.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/921/head:pull/921

PR: https://git.openjdk.java.net/skara/pull/921