Fixed replies to go on the correct side of the diff in Differential.

Summary:
Replies to comments went always to the left side of the diff. There
was a confusion between 'is_new' and 'on_right'. Database wants the
information if the reply is on the left or on the right. The database
does not care if the comment is a reply or an original comment.

The code looks a bit confusing, because the database field is called
'isNewFile' and that is used to determine, which side the comment
applies to.

Test Plan:
Tested that every of combination of new comment/reply to the left/right side
worked when editing and after submitting the comment.

Reviewed By: jungejason
Reviewers: jungejason
CC: epriestley, jungejason
Differential Revision: 148