The DiffSet model has been updated to be able to contain several

DiffCommits in addition to FileDiffs. The DiffCommit model

represents a single commit in distributed version control system

(DVCS). Each DiffCommit has one or more FileDiffs that belong to

it (as well as the DiffSet).
Add an evolution to add a foreign key on the FileDiff model to point

to an optional DiffCommit model. Also add an evolution to add a

RelationCounterField to the DiffSet model that counts the number

of DiffCommits it currently has.
Update the reviewboard.diffviewer.admin module to add the ability to

view individual DiffCommits. The DiffCommits can be as inline

models of the DiffSet model and have FileDiff as an inline model.

Ran unit tests.

Applied evolution successfully.