GFM mode bold fontification incorrect for within-token underscores #525
Description
Expected Behavior
This issues is essentially the same as #448 but for bold fontification via double underscore (__). In GFM mode, like single asterisk and underscore, within-token double asterisk should fontify as bold while within-token double underscore should not fontify, i.e.
**strong**
st**ro**ng
**str**ong
str**ong**
**still strong**
**still str**ong
sti**ll strong**
sti**ll str**ong
__strong__
not__str__ong
__not__strong
not__strong__
__still strong__
no__t strong__
__not str__ong
no__t str__ong
should be (at least partially) fontified as their respective text suggests.
Actual Behavior
Like double asterisk, double underscore fontifies in all of the above cases.
Note: See also #524, which observes a bug when just one character is surrounded by single asterisks or underscores.
Software Versions
markdown-mode version 2.5-dev (installed via MELPA)
GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14) of 2020-03-26, modified by Debian
Ubuntu 20.04.1 LTS