download_baselines.py : only set svn properties of files whose content has changed

and while I was at it, sort the filenames before acting on them, to make the output easier to read

BUG=http://code.google.com/p/skia/issues/detail?id=618
Review URL: https://codereview.appspot.com/6223061

git-svn-id: http://skia.googlecode.com/svn/trunk@4031 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed