bug#46485: 28.0.50; Recent change to count-lines broke gnus-cite faces

[Kévin Le Gouguec]

Hello,

Since a few days ago, Gnus sometimes applies gnus-cite faces to
paragraphs below the actual citations.  For example, save the attached
article somewhere, then, from emacs -Q:

- M-x gnus-no-server
- G f /path/to/repro.mbox
- RET RET

There are a few things that seem to go wrong with fontification of
citations; for example, the line that starts with "That is a concern" is
erroneously fontified with gnus-cite-1.

I've tried to bisect this, and Git points out 2021-02-07 "Use
`line-number-at-pos' in `count-lines'" (7c5938ad7d).

When time permits, I'll start looking into what causes the bug, and
maybe whip up a test case; I figure someone more familiar with the code
might easily beat me to it though, so I'm reporting this before having
fully figured it out.

repro.mbox
Description: application/mbox

In GNU Emacs 28.0.50 (build 13, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, 
cairo version 1.16.0)
 of 2021-02-08 built on my-little-tumbleweed
Repository revision: 9a698da7dea51a59aa9ddfb71887ac6b865883dc
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: openSUSE Tumbleweed

Configured using:
 'configure --with-xwidgets --with-cairo'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBXML2 M17N_FLT MODULES NOTIFY INOTIFY
PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
XWIDGETS GTK3 ZLIB

Important settings:
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix