Without this, multiple spaces or tabs in the commit message aren't
preserved and things like tables don't align properly.
As part of adding the CSS rule, we had to cuddle the content
with the <div> to not introduce leading and trailing whitespace.
The "addbreaks" filter was also removed because it would insert
an additional newline, effectively double spacing content.
Differential Revision: https://phab.mercurial-scm.org/D113
21ad83cca7