The DiffCommitResource now accepts the original commit IDs of a commit

when creating. These IDs can be used to show the evolution of a commit

history over time, e.g., when two commits get squashed into one commit

or a commit is amended.
The ValidateDiffCommitResource has also been updated to accept the

required fields.

Ran unit tests.
Used https://reviews.reviewboard.org/r/8278/ to upload a rebased

  patch with rbt and saw the relationships in the database.