Unique list of authors, but preserving order.

def unique_authors(self, limit):
        """Unique list of authors, but preserving order."""
        seen = set()
        if limit == 0:
            limit = None
        seen_add = seen.add  # Assign to variable, so not resolved each time
        return [x.author for x in self.sorted_commits[:limit]
                if not (x.author in seen or seen_add(x.author))]