git cl description: don't bother if description diff is trailing whitespace.

BUG=700239
R=iannucci@chromium.org

Change-Id: Ica4567288c1cd83f72cce5d10c523270b69c36d4
Reviewed-on: https://chromium-review.googlesource.com/455739Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cl.py

@@ -4322,7 +4322,7 @@ def CMDdescription(parser, args):
   else:
     description.prompt()
 
-  if cl.GetDescription() != description.description:
+  if cl.GetDescription().strip() != description.description:
     cl.UpdateDescription(description.description, force=options.force)
   return 0