RFR: vcs: support setting dates for tags

[Erik Helin]

Hi all,

please review this patch that allows setting the date for tag objects.

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

Thanks,
Erik

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Commit messages:
 - vcs: support setting dates for tags

Changes: https://git.openjdk.java.net/skara/pull/735/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=735&range=00
  Stats: 40 lines in 4 files changed: 32 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/735.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/735/head:pull/735

PR: https://git.openjdk.java.net/skara/pull/735