Parse the output of filenames_diff_cmd.

def parse_diff_filenames(diff_files):
        """Parse the output of filenames_diff_cmd."""
        # ?   .gitignore
        # M  0.txt
        files = []
        for line in diff_files.splitlines():
            line = line.strip()
            fn = re.findall('[^ ]+\s+(.*.py)', line)
            if fn and not line.startswith('?'):
                files.append(fn[0])
        return files