Review requests that contain commit history will now show the list of

commits on the current diff revision. The changes to the commits in

diff revisions are now reflected in change descriptions.
The BuiltinLocalFieldMixin has been modified to allow subclasses to

override its default behaviour (where the review_request_details are

replaced if it is a draft and the field does not exist on the draft).

This was changed because the new DiffCommitListField needs to display

the draft commits and the default behaviour would prevent that.

Tested the following manually:

Created a review request with commit history. The commit list field

   was displayed in the draft view.
Published the review request. The commit list field was displayed in

   the public review request.
Uploaded a new diff with history. The draft view reflected the

   changes in the commit list field.
Published the review request. The change description reflected the

   changes to the commits on the review request.
Published the review request without uploading a new diff. The commit

   list field did not change and it did not appear in the change

   description.
Updated the review request using a new diff with history. The change

   description was rendered properly.
Updated the review request using a squashed diff. The commit list

   field disappeared from the review request and the change description

   showed the removed commits.
Updated the review request using a a diff with history. THe commit

   list field re-appeared in the review request and the chagne

   description showed the added commits.