Use pre-wrap for diff code in discussion view

This prevents merge request comments from being hidden in discussion view.  See #2017.

### Before

![diff_comment_before](https://dev.gitlab.org/Okada/gitlabhq/uploads/e6d24987b8d88f4e8a5883645d569b56/diff_comment_before.png)

### After

![diff_comment_after](https://dev.gitlab.org/Okada/gitlabhq/uploads/fc035f9fc44718dc8424d3f8b54f0953/diff_comment_after.png)

This change doesn't affect the "Changes" tab:

![diff_view_comment](https://dev.gitlab.org/Okada/gitlabhq/uploads/9975d4c3feac629d6a0d76b5a0ef43d8/diff_view_comment.png)

cc @sytse

See merge request !1700