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

patch.)
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.