bug#20871: 25.0.50; fill-single-char-nobreak-p does not recognize a single-letter word when it is preceded by an open paren

[Eli Zaretskii]

> From: Marcin Borkowski <mbork@mbork.pl>
> Cc: 20871@debbugs.gnu.org
> Date: Mon, 15 Jan 2018 06:30:20 +0100
> 
> I attach the patch (which is made by squashing both the commits I made).

Thanks, this LGTM.  Please push to master.