node_diff() only diffs modified existing values, ignores newly added or removed ones

Patch attached.
Here's a screenshot: https://skitch.com/katbailey/fk9e6/diff-new-and-removed WIthout this patch those changes wouldn't show up.

Yay - just saw it made it into the 2.0 release, awesome! :-)
What's not awesome is that I hadn't tested it properly with preview changes, and when I did the changes involving removed field values were not showing up. The problem is that the new version of the node still has the $langcode key set for the field, even though it's an empty array. This gets blown away when the node gets saved, which is why it works fine when you do a full revision diff.
Here's a follow-up patch that checks that it's not empty.