Allow users to set a git config entry which stops
gclient from messing with remote.origin.url. Useful
for using experimental repos.

Review URL: https://chromiumcodereview.appspot.com/14731015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@199532 0039d316-1c4b-4281-b951-d872f2087c98