Add show_gm_changes script: show change revisions and hashes for a GM

BUG=skia:1920
R=epoger@google.com

Author: borenet@google.com

Review URL: https://codereview.chromium.org/213833017

git-svn-id: http://skia.googlecode.com/svn/trunk@14015 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed