def get_changelog(self, previous_revision=None, max_revisions=None):
raise NotImplementedError<EOL>
Get the changelog text since the given revision.

        If previous_revision is not an ancestor (for example, the last release
        was from a different branch) you should still return a meaningful
        changelog - perhaps include a warning, and give changelog back to the
        last common ancestor.

        Args:
            previous_revision: The revision to give the changelog since. If
            None, give the entire changelog.

        Returns:
            Changelog, as a string.