The current diff viewer shows both the old and new view for every diff. 

Sometimes, we want to just look at the new view and not the whole diff with 

the existing code. Adding the feature to hide the old view and only show the 

new view will allow users to have an uncluttered view.
We added a [Show/Hide] toggle to the revision-row on the diff viewer. This

button will toggle between showing both views and only showing the new view. 

The button toggles a show-new-only class which show or hide the old view with 

the CSS display: none property. If the line contain comments, the comment 

flags are detached from the old view's column and appended to the new view's

column using jQuery's detach() and appendTo() function, vice versa.

Testing Done:

Wrote onShowNewClicked and On show new only in

diffReviewableViewTests.es6.js.

Tested following behaviour in browser: 

Added comments before toggling, able to view in both views
Added comments after toggling, able to view in both views