use the git_revision property of Changes to know the git rev

honor a "git_revision" change property to know which git revision to test.

An associated change in svn_buildbot.py includes this property the addChange call.

I've tested this with a local setup created with briad, using the post-commit svn_buildbot.py script (very slightly hacked since I didn't have a local copy of the svn repo). I've also tested builds through the web (which won't have the git_revision property, and just run the HEAD of a branch).