Switch back to using 'diff-tree'

Thanks to Alistair for pointing out how the raw ('-r') option works,
which I'd utterly missed. This is preferable to 'show' since it's a
lower level command which ought to be faster. It also involves less
output muging on our part since it doesn't output the bonus empty
line my previous format hack generated.

Change-Id: If48af6f6f11656e08d63e6463071ecfea87a1956