svndiff.py: add ability to compare before-and-after JSON files, not just raw images

This should complete step 3 of https://goto.google.com/ChecksumTransitionDetail !

R=borenet@google.com

Review URL: https://codereview.chromium.org/19112002

git-svn-id: http://skia.googlecode.com/svn/trunk@10113 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed