Prevent review requests created with history from adding regular diffs

Review Request #9852 — Created April 3, 2018 and submitted — Latest diff uploaded


Review Board


We now have a notion of a review request that is created with history
support. A review request created with this flag cannot have a regular
diff attached to it. Instead, an empty DiffSet must be created and
commits attached to that. (Support for which is coming in another

This option is now specified at review request creation time and cannot
be changed once the review request is created (i.e., after save() is
called and the primary key has been assigned). It can be specified via
a new field on the review request resources which defaults to the old
(i.e., no history) behaviour.

Ran unit tests.