The review diff comment API can now validate the

extra_data.base_commit_id and extra_data.condensed_diff fields if

they are provided. Utility properties have been added to the

reviews.models.diff_comment.Comment model to facilitate getting and

setting the base commit ID and condensed diff status of the associated

file diff. These will be updated by JavaScript through the API so that

the correct diff will be rendered in reviews.
Unit tests have been added to verify the validation of these new API

fields.

Ran unit tests.