RFR: Add ReadOnlyRepository.annotate(Tag t) method

[Erik Helin]

Hi all,

please review this patch that adds the `ReadOnlyRepository.annotate(Tag t)` method that can be used to get more information for annotated tags (git) or just tags (hg).

Thanks,
Erik

## Testing
- `make test` passes
- Added three new unit tests

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Commits:
 - a8bd1e7c: Add ReadOnlyRepository.annotate(Tag t) method

Changes: https://git.openjdk.java.net/skara/pull/242/files
 Webrev: https://webrevs.openjdk.java.net/skara/242/webrev.00
  Stats: 167 lines in 6 files changed: 167 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/242.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/242/head:pull/242

PR: https://git.openjdk.java.net/skara/pull/242