Don't show "Show changes" when there is only one diff revision
Added logic to review_detail.html so that if the past revision number of a diff is 0 (a diff has been added after a review request has been published) a "Show changes" link would not be displayed.

Reproduced the bug by publishing a review request, then adding a diff and verifying that the "Show changes" link does not appear. Added a second diff to the same review request and verified that the "Show changes" link does appear as it should for the second diff.