RFR: notifier: add commit comment notifier

[Erik Helin]

Hi all,

please review this patch that adds a new notifier - the commit comment notifier. The notifier is simple, it will add a
comment to a pushed commit with links to the pull request and to fixed issues. This allows people to browse the history
online and use the links in the comment to directly go to the review and/or issues (this already possible via CLI with
the `git info` command).

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

Thanks,
Erik

-------------

Commit messages:
 - notifier: add commit comment notifier

Changes: https://git.openjdk.java.net/skara/pull/644/files
 Webrev: https://webrevs.openjdk.java.net/skara/644/webrev.00
  Stats: 297 lines in 5 files changed: 296 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/644.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/644/head:pull/644

PR: https://git.openjdk.java.net/skara/pull/644