Add a new gclient-new-workdir script which clones an existing gclient working directory much like git-new-workdir, but takes into account all sub projects as well.

BUG=none

Review URL: https://codereview.chromium.org/52663008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@232992 0039d316-1c4b-4281-b951-d872f2087c98