Add a model representation of a commit in a series

Review Request #9609 — Created Feb. 9, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
b8f56a6...

Reviewers

This is the first of a series of commits to introduce support for commit
series review. This patch adds the necessary database model
(DiffCommit) and updates the existing models to introduce
relationships.

Ran unit tests.
Applied evolution.