Add the a diffviewer.Commit model. Each Commit belongs to a

DiffSet (which may have many commits) and may have many FileDiffs.

However, an old-fashioned review request (of squashed changes) will

not have any Commits associated with it.
Add a foreign key on the FileDiff model which links it to a

Commit. Every FileDiff still corresponds to a DiffSet, but they

now can optionally correspond to a Commit.
Add a RelationCounterField on the DiffSet model to keep track of

the number of Commits corresponding to the DiffSet. This way, the

DiffSet can easily be checked if it is a multi-commit review request

or not.
Add evolution (new_commit_fields.py) for new fields on FileDiff

and DiffSet.
TODO:

- Add an API endpoint so that multiple commits can be attached a review

  request