Refactor out common diff generation code

Review Request #3864 — Created Feb. 11, 2013 and submitted — Latest diff uploaded

Information

RBTools
master

Reviewers

Refactor out common diff generation code.

The post and diff commands were using seperate repeated code for
generating diffs. This common code is now a function which the
commands can call to generate the diff.
Generated a couple diffs using rbt diff, and posted this review request using rbt post.