Add the DiffCommitResource and DraftDiffCommitResource resources.

The DiffCommitResource is a resource for retrieving the information

about the DiffCommits under a particular DiffSet. The

DraftDiffCommitResource inherits from the DiffCommitResource and

modifies it to allow for the creation of DiffCommits and associated

FileDiffs. The DraftDiffCommitResource handles the uploading of

FileDiffs for DiffCommits instead of the DiffSetResource.
The DiffSetResource POST 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

DiffCommit 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 DiffCommit instances.
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 DiffCommits.
Add the DiffCommitManager model. It inherits from the

DiffProcessorMixin.
Add a WebAPI capability to show that the server supports review

requests with commit histories. This allows new versions of rbt that

support commit histories to create review requests with history by

default if the tool supports it.
Add new unit tests for the web API. The tests include:

 - test_filediff, which tests FileDiff filtering of a DiffCommit

 - test_draft_diff_commit, which does the base API tests for the

DraftDiffCommitResource

 - test_diff_commit, which does the base API tests for the

DiffCommitResource

Unit tests pass.
Did the following manual testing:

Created a review request via rbt post. It was automatically

   created with history. (There was one commit under the draft).
Able to read the commit list and single commit via the API.
Able to filter the FileDiff list resource.
The FileDiff item resource was not filtered.
Able to publish, discard, and delete the review request.
Able to use all the public API endoint functionality of the

   published RR anonymously