bug#27391: 25.2.50; utf-8 coding cookie is not applied on some specific markdown file

[Eli Zaretskii]

> From: vincent.belaiche@gmail.com (Vincent Belaïche)
> Cc: Vincent Belaïche <vincent.belaiche@gmail.com> 
> Date: Fri, 16 Jun 2017 16:08:09 +0200
> 
> Attached is the file causing the issue. Recipe is just to visit the file
> with emacs -q, and you see that the encoding is not taken.

Your fancy comment causes this: remove the leading '[' and the problem
goes away.  Looks like regex-quoting that somehow misfires.