Fix issue where all rows in "Revision Update History" render the same base

Summary: We were not correctly updating $diff as we iterated through the loop.

Test Plan: Viewed a revision several diffs that had differing base revision.

Reviewers: fratrik, btrahan

Reviewed By: fratrik

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1523