For the used diff engines an empty/new page is equivalent
to an empty line. When adding content that includes an
empty line the content will be split into two parts. The
interpreted diff will then consist of one addition, one
copied line and another addition.
Is the ping in the first addition and the signature in the
second addition the DiscussionParser does not send mentions.
The patch introduces a special case when interpreting diffs
if content is added to empty content. This will also skip
execution off diff engines since the interpretation is trivial.
Bug: T155998
Change-Id: Id5e44bc3245940c1f77c80f036db637756542552
