Add test of the changelog() method

The test is written to support both the CGit and the JGit implementation.  Unfortunately, the JGit implementation returns an empty string, while the CGit implementation returns the expected results.

Rather than lose this test, I marked it as not implemented in JGit.  That prevents it from being reported as a failure in JGit, and can easily be removed once the JGit implementation output is adapted to match the CGit output.