while it is possible to diff even between arbitrary commits, it always compares in date order as per answer on https://answers.atlassian.com/questions/194632/sourcetree-how-to-do-a-diff-in-the-opposite-direction

in order to better see merge results, it would be an absolutely outstanding feature to be able to flip the direction of the diff comparision. Apart from git's commandline there doesn't seem to be any good solution out there and is the only thing that I am missing from SourceTree - everything else is perfect!