We now require that diffs for an entire series of commits be validated

before posting to the DraftDiffCommitResource. Validation is performed

by making a series of POST requests to the

ValidateDiffCommitResource, each of which returns validation data to

be included in the next request. The validation data from each request

to the validation resource must then be used to POST to the

DraftDiffCommitResource.
This ensures that we do not upload broken diffs and also allows us to

upload more complex commit series. Previously, only linear commit series

where each commit modified different files were capable of being

uploaded. Now all linear commit series can be uploaded.

Posted /r/9609/ through to this commit to a single review request.

Ran unit tests.