Add the DiffCommitResource and DraftDiffCommitResource resources.

The DiffCommitResource is a resource for retrieving the information

about the Commits under a particular DiffSet. The

DraftDiffCommitResource inherits from the DiffCommitResource and

modifies it to allow for the creation of Commits and associated

FileDiffs. The DraftDiffCommitResource handles the uploading of

FileDiffs for Commits instead of the DiffSetResource.
Register the Commit model for both DiffCommitResource and

DraftDiffCommitResource in the API. They are item child resources of

the DiffSetResource and DraftDiffSetResource resources,

respectively.
The DiffSetResource.create method now takes an extra optional field,

create_empty, that determines if the DiffSet that is created is

created without uploading a .diff file. This is used in the creation

of review requests with commit histories.
The FileDiffResource and DraftFileDiffResource classes now filter

their results based on the commit query parameter. If no commit

query parameter is given, only those FileDiffs unattached to a

Commit will be return. This filtering is only applied to the list

API, not the item API.
Add a new form, the UploadCommitForm, for handling the processing of

data for creating new Commit instances. It is very similar to

diffviewer.forms.UploadDiffForm and may need to be refactored.
Refactor behaviour for uploading FileDiffs from the DiffSetManager

to a new class, the DiffProcessorMixin. This mix-in handles the

initial processing of .diff files, their comparison (for header vs

source files), generating file lists, and creating actual FileDiffs

from upload (it calls the child class's implementation of

create_from_data).
The DiffSetManager now has a create_empty method for creating

DiffSets without FileDiffs for usage with Commits.
Add the CommitManager model. It inherits from the

DiffProcessorMixin.
Update the validate_diff endpoint to use named parameters for

DiffSetManager.create_from_upload because of the refactoring.
Add a WebAPI capability to show that the server supports review

requests with commit histories.
TODO:

  - Unit tests forthcoming.

  - Manual testing information forthcoming.

Ran unit tests.

Able to create a review request with a single commit of history via

RBTools (see: https://reviews.reviewboard.org/r/6787/).