RFR: forge: support commit comments

[Erik Helin]

Hi all,

please review this patch that adds the possibility to add a comment on a particular commit to `HostedRepository` via
the method `addCommitComment`. To start out with I've only added support for adding a general comment about a whole
commit, not a comment for a particular line in a commit.

The comments for a commit can also be listed with the method `commitComments`.

Testing:
- Not that much, later patches will build upon the APIs introduced in this patch

Thanks,
Erik

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Commit messages:
 - forge: support commit comments

Changes: https://git.openjdk.java.net/skara/pull/639/files
 Webrev: https://webrevs.openjdk.java.net/skara/639/webrev.00
  Stats: 218 lines in 7 files changed: 211 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/639.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/639/head:pull/639

PR: https://git.openjdk.java.net/skara/pull/639