Refactor logic out of UploadDiffForm

Review Request #4231 — Created June 11, 2013 and submitted — Latest diff uploaded

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Review Board
master

Reviewers

Refactor logic out of UploadDiffForm

All of the logic for taking a diff file and creating a DiffSet was in
UploadDiffForm. This change moves it out into a manager, so that I'll be able to
use the same code for creating DiffSets from diff data fetched from an upstream
repository.
Ran unit tests.