v8: update make-v8.sh to use git

google build tool gclient doesn't support
svn anymore. Updating v8 build script to use
git instead.

PR-URL: https://github.com/nodejs/node/pull/9393


Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>