Refactor the DiffCommitListResource into a mixin to allow commmit

behvaiour to be shared between the original DiffCommitListResource

and the new ValidateDiffCommitResource. This new resource is

responsible for validating a commit (the diff and the metadata) before

it is uploaded and added to the review request.
This new validator is used to validate all commits as they are

uploaded. This replaces the old behaviour where only the diff was

validated (using the ValidateDiffResource) but none of the metadata

was.

Ran unit tests.
Able to upload a review request with history and have its commits

validated.