We were not handling mnemonics vs markup right
in all cases. Rewrite the _-stripping code to
do it during the link parsing, instead of as
a separate function. This avoids the issue of
stripping _ from attribute names in markup.
Add tests.
Fixes: #3706
dboles: cherrypick of mclasenʼs 875a92b95f
only notable difference is expected output in test – unsure why?
6ef130d40b