Fix: Review comments do not show up for unchanged files

When comparing patch sets and some comment was put in one side,
that comment is not shown if there is not any code changed between
the two patch sets, Gerrit only say "No Differences", thus there's a
risk that this comment would been ignored.

Bug: issue 2122
Change-Id: Ib56475c2bc0114feffe64883708652baa7be9d1e