[Fix JENKINS-16737] and [Fix JENKINS-10434] - no exception if author not found

Return unknown user rather than throwing exception in GitChangeSet

Refer to [JENKINS-16737] and [JENKINS-10434] for two cases where a
GitChangeSet cannot find an author and throws a RuntimeException when
it would be much better to report an unknown user and allow execution
to continue.

Added a Polish character to the accented character test

Use more diacritics in the latin accented character test

Use parameterized GitChangeSet test to better cover cases