Add svndiff tool: runs skdiff over all pending changes in SVN checkout

This is needed for 
https://goto.google.com/HowToLandSkiaChangesThatChangeWebkitLayoutTests ,
and will be generally useful beyond that too.
Review URL: https://codereview.appspot.com/6256052

git-svn-id: http://skia.googlecode.com/svn/trunk@4052 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed