In "git cl description", skip updating when unchanged

BUG=473423

Review URL: https://codereview.chromium.org/1058793004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294671 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -1575,7 +1575,8 @@ def CMDdescription(parser, args):
     DieWithError('This branch has no associated changelist.')
   description = ChangeDescription(cl.GetDescription())
   description.prompt()
-  cl.UpdateDescription(description.description)
+  if cl.GetDescription() != description.description:
+    cl.UpdateDescription(description.description)
   return 0