Use parsed revisions for Plastic diffs.

Review Request #5165 — Created Dec. 24, 2013 and submitted — Latest diff uploaded

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RBTools
master

Reviewers

Use parsed revisions for Plastic diffs.

This is a very trivial refactor. For the most part, this just combines the old
changenum_diff and branch_diff methods and renames some methods to be internal.

None. I'm planning to add unit tests for plastic later on.