Create gitdeps.py script.

This emulates 'gclient' behaviour, but for 'deep' GIT dependencies. It will be necessary once Chrome has transitioned to a pure GIT checkout, as we will be unable to roll DEPS beyond that point.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/123660043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2274 15e8cca8-e42c-11de-a347-f34a4f72eb7d