1) don't save empty changelists to begin with. This guards against an accidental or canceled "gcl change" creating an empty file.
2) create a "clearempties" command which clears all changelists that don't have any files in them.

Review URL: http://codereview.chromium.org/160139

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@22538 0039d316-1c4b-4281-b951-d872f2087c98