Add a with_diff= option to TestCase.create_diffcommit.
Review Request #2231 — Created Sept. 23, 2021 and updated
`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`. Testing Done: Unit tests pass. Made use of this new argument in an upcoming change. Reviewed at https://reviews.reviewboard.org/r/11759/