TestCase.create_diffcommit() takes in diff files and runs them through

the diff parsing and FileDiff creation code, which is sometimes

overkill (or completely unwanted) for a test.
This change adds a new with_diff= argument to this method, which can

turn off this behavior. When passing with_diff=False, the DiffCommit

will be created in the database directly, without any additional

processing, and without a FileDiff.

Unit tests pass.
Made use of this new argument in an upcoming change.