Fixed large comments messing with diff table widths

## What does this MR do?

Fixes the width of the diff table so that the content inside it can't increase or decrease the width of the table cells.

## Screenshots (if relevant)

![Screen_Shot_2016-09-12_at_14.44.44](/uploads/2ca483399d867153e6ae608360ffea5f/Screen_Shot_2016-09-12_at_14.44.44.png)

## What are the relevant issue numbers?

Closes #20694

See merge request !6309