Changed code blocks appeared very weird in the field diffs shown in

change descriptions. There was a lot of padding and blank lines that

shouldn't have been there, and the borders were unnecessarily being

shown.
This change fixes up the styling to make the code blocks look correct,

and to fix blank lines so that they show up as part of the code block.

Tested with several field diffs, with and without code blocks. Saw that

the code blocks look correct now.