libvbucket: Add convenience function to inspect diff

Change-Id: I28b9f466b7c0f76e55cd5a6a41c3e04bcaa0210e
Reviewed-on: http://review.couchbase.org/33741
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>