The DiffProcessorMixin has been refactored into two classes: the

DiffManagerBase class, which is a base class for the creation of

DiffSets and DiffCommits; and the DiffProcessor, which handles

the creation of FileDiffs from raw diffs.
The logic is separated into these two classes so that in a future

patch the DiffCommitManager can provide a file existence checker

based on the currently existing DiffCommits in a DiffSet for diff

file validation.

Ran unit tests.