Use the code in git_cl.py, since it was the more evolved. Add documentation
and clean up the structure along the way.

This makes it possible to easily reuse the generic subcommand handling code.

As a first step, only git_cl.py is using it. Eventually, gclient and gcl could
be switch over.

R=iannucci@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23250002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@218072 0039d316-1c4b-4281-b951-d872f2087c98