Move git_cl back into depot_tools.

README.testing

 Most of the tests require a local Rietveld server.
 
 To set this up:
 Method 1: Let the presubmit script do the work for you.
    $ git cl presubmit
 
 Method 2: Manual.
 1) Check out a copy of Rietveld:
      $ svn checkout http://rietveld.googlecode.com/svn/trunk/ rietveld
    (Use git-svn if you must, but man is it slow.)
 2) Get the Google App Engine SDK:
    http://code.google.com/appengine/downloads.html
 3) To run Rietveld you will need Django 1.0, which is not included
    with the App Engine SDK.  Go to http://www.djangoproject.com/download ,
    download a Django from the 1.0 series (it's in the sidebar on the right),
    untar it, then
      $ export PYTHONPATH=`pwd`/Django-1.0.4
 4) Run Rietveld:
      $ /path/to/appengine/sdk/dev_appserver.py /path/to/rietveld
    (If using one of the App Engine launchers, be sure to use port 8080
    for this project.)
 
 And then, finally, run the tests.