RFR: 2163: Add API to Repository for working "git notes"

[Erik Duveblad]

Hi all,

please review this patch that adds the methods `addNote`. `notes` and `pushNotes` to `Repository.java` to allow Skara code to work with [git notes](https://git-scm.com/docs/git-notes). This will be used in another patch to implement a "git note"-notifier (see https://bugs.openjdk.org/browse/SKARA-2164). 

I opted for a fairly minimal API here even though the `git notes` command exposes many more features. I also decided to add the `pushNotes` method although callers could use the `push(String refspec, URI uri)` variant because I don't like the notes refspec to "leak" out into other parts of the code.

Also added a couple of unit tests.

Thanks,
Erik

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Commit messages:
 - 2163

Changes: https://git.openjdk.org/skara/pull/1607/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1607&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2163
  Stats: 121 lines in 4 files changed: 121 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1607.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1607/head:pull/1607

PR: https://git.openjdk.org/skara/pull/1607