RFR: 2164: Add git notes notifier

[Erik Duveblad]

Hi all,

please review this patch that adds a new notifier that records the link to a commit, the link to the review and ev. links to issues in a [git note](https://git-scm.com/docs/git-notes). This will make it possible to view this information using e.g. `git log` or `git show` _without_ having these URLs in the commit message itself. Git notes are also modifiable (they are just blobs referred to by a ref), so they can be updated if ever change the scheme or domain for these links.

I also added two unit tests for testing the new notifier.

Thanks,
Erik

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Commit messages:
 - 2164

Changes: https://git.openjdk.org/skara/pull/1608/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1608&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2164
  Stats: 312 lines in 7 files changed: 310 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1608.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1608/head:pull/1608

PR: https://git.openjdk.org/skara/pull/1608