+activereviews times out showing hundreds of merge proposals

Branch:+activereviews times out with very high non-SQL time sometimes when showing several hundred proposals. We probably want to add batching to the page eventually, but the current version should not be even half as slow as it is.

eg. https://code.launchpad.net/~openerp/openobject-addons/trunk/+activereviews is showing ~650 MPs at present, and takes 8 seconds to render with OOPS-bc2ffc5a1fc4d8a814513df92ad0e2f2. It has some reasonably large diffs, conflicts, and diffstats which are being loaded unnecessarily, but nothing that should take 7 seconds.