Previously, the DiffSetManager would create DiffSets via the 

model.Manager.create method and then optionally save the generated

model. However, this method will always end up saving the model to the

database. Now, we explicitly instantiate the model in the manager

and optionally call the save method on it.

Ran unit tests.
Uploaded a review request that had its diff validated. Verified that

only one DiffSet was created in the database.