RFR: vcs: handle lightweight tags in Git

[Erik Helin]

Hi all,

please review this patch that makes `GitRepository.annotate` handle lightweight tags gracefully.

Testing:
- [x] Added a unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - vcs: handle lightweight tags in Git

Changes: https://git.openjdk.java.net/skara/pull/670/files
 Webrev: https://webrevs.openjdk.java.net/skara/670/webrev.00
  Stats: 38 lines in 2 files changed: 35 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/670.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/670/head:pull/670

PR: https://git.openjdk.java.net/skara/pull/670