Redo relative url implementation per maruel's comments here:
http://codereview.chromium.org/500015

Instead of adding an option, implement relative urls differently for
git urls.

Review URL: http://codereview.chromium.org/546022

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@36126 0039d316-1c4b-4281-b951-d872f2087c98