This method starts outputs the CSS for the HTML message.
SVN::Notify::HTML::ColorDiff adds extra CSS to its output so that it can
nicely style the diff.
Reads the diff data from $diff_file_handle and prints it to
$out_file_handle for inclusion in the notification message. The diff is
output with nice colorized HTML markup. Each line of the diff file is escaped
If there are any diff filters, this method will do no HTML formatting, but
redispatch to SVN::Notify::output_diff. See
Writing Output Filters for details on