RFR: 2152: Use full name for tags if possible

[Erik Duveblad]

Hi all,

please review this patch that makes the `/tag` commit command use the user's full name (if possible) for the author and committer name field for the Git tag it creates. This is the same convention we use for the author/committer name and e-mail metadata in the commits (it was just a mistake on my part that `contributor.username()` was used in the original code for the `/tag` commit command).

### Testing
- [x] Added a new unit tests that fails without this patch but passes with it

Thanks,
Erik

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Commit messages:
 - 2152

Changes: https://git.openjdk.org/skara/pull/1603/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1603&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2152
  Stats: 52 lines in 2 files changed: 51 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1603.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1603/head:pull/1603

PR: https://git.openjdk.org/skara/pull/1603