Make 'gcl diff emptychange' show nothing if given change, |emptychange|, doesn't contain files.
Make 'gcl diff emptychange' show nothing if given change, |emptychange|, doesn't contain files.

BUG=27243
TEST=Create change which contains no files and confirm 'gcl diff createdchange' doesn't output anything.

M      gcl.py

Review URL: http://codereview.chromium.org/388001

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@31551 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools