Clone GitHub mirrior into wordpress-trunk alongside SVN checkout

Hacking on WordPress is much easier using Git, as commits can be made and pushed up to a fork on GitHub for collaboration and generating patches to submit to Trac for application to SVN. So this pull request adds a `.git` clone to live alongside the `.svn` checkout in `wordpress-trunk`.

When provision script runs, the Git clone is updated via `git reset --soft origin/master` because SVN may very well be ahead, and so we just want to advance the master commit HEAD to the furthest point, but not overwrite the more up-to-date update from SVN.