RFR: vcs: handle nested repositories

[Erik Helin]

Hi all,

please review this patch that makes `Repository.get` (and
`ReadOnlyRepository.get`) able to handle nested repositories.

Testing:
- `make test` passes on Linux x64
- Added three new unit tests

Thanks,
Erik

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Commit messages:
 - vcs: handle nested repositories

Changes: https://git.openjdk.java.net/skara/pull/496/files
 Webrev: https://webrevs.openjdk.java.net/skara/496/webrev.00
  Stats: 94 lines in 2 files changed: 90 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/496.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/496/head:pull/496

PR: https://git.openjdk.java.net/skara/pull/496