RFR: 1859: Add "tags only" option to MirrorBot

[Erik Helin]

Hi all,

please review this patch that adds an option to the `MirrorBot` to mirror only tags from one repository to another. I also added some additional logging to `MirrorBot` and cleaned up the logic a bit for selecting the "mode" (tags, mirror, selected branches) that the `MirrorBot` uses.

### Testing
- [x] Added one additional unit test

Thanks,
Erik

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Commit messages:
 - 1859

Changes: https://git.openjdk.org/skara/pull/1493/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1493&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1859
  Stats: 163 lines in 7 files changed: 134 ins; 10 del; 19 mod
  Patch: https://git.openjdk.org/skara/pull/1493.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1493/head:pull/1493

PR: https://git.openjdk.org/skara/pull/1493