Ordering revisions by date isn't necessarily the right thing to do on diff view

Currently when showing a diff the r1 and r2 are ordered by date, so that the earliest revision will be to the left, and the latest to the right.

Consider the following example where a customer is diffing a file from two different branches:

Here they would rather have r1 = 1.48.4.4 and r2 = 1.53 considering that 1.48.4.4 is derived from an earlier revision.