Update disagreements calculation.

Previously, this calculation only considered a positive vote when a core
team member gave a negative vote.  It now:

1) Also considers the opposite case: a negative vote overridden by a
positive vote

2) Considers vote order.  It only counts as a disagreement against a
reviewer if the core vote comes *after* the review in question.

Change-Id: Ia4423af05135d7dd26a6bdae667d54eae538f774