Enhance RB.Diff to be able to create new diffs.

Review Request #4303 — Created July 7, 2013 and submitted — Latest diff uploaded

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Review Board
master

Reviewers

Enhance RB.Diff to be able to create new diffs.

The existing diff model didn't do very much, and was really meant for dealing
with existing diffs. This change adds some attributes and methods that allow it
to be used to create new diffs.
Used this in another change to create a diff from javascript.