The main logic of the DiffSetManager has been refactored into two

parts:

reviewboard.diffviewer.filediff_creator, which handles the

  actual processing of diff files into FileDiffs; and
DiffManagerBase, which provides a base implementation of

create_from_upload for both the DiffSetManager and the

DiffCommitManager.

The new DiffCommitManager is able to create DiffCommit objects from

either uploaded files or from data, just as the DiffSetManager can for

DiffSet objects.
This patch also adds DiffSetManager.create_empty to create empty

DiffSets which will contain DiffCommits.

Ran unit tests.