add commit message as a file to the changes browser

- similar to gerrit's web UI the commit message is now displayed as
  a file in the changes browser view
- comments can be viewed and added just like with other files
- resolves #87
better support for merge commits CommitMessageFormatter

- support commits with more than 2 parents
Merge pull request #89 from forrert/changes-browser/commit-message

add commit message as a file to the changes browser