RFR: 1859: Add "tags only" option to MirrorBot [v2]

[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

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Reviewer feeback

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Changes:
  - all: https://git.openjdk.org/skara/pull/1493/files
  - new: https://git.openjdk.org/skara/pull/1493/files/9a85b09f..e84bc944

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1493&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1493&range=00-01

  Stats: 210 lines in 3 files changed: 201 ins; 0 del; 9 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