A customer is looking for the ability to view all changes in a commit, not only in respect to previous version, but also with a specific previous revision.

A global "view changes", Trac has something similar that is very powerful:

http://trac.edgewall.org/changeset?old_path=%2Ftrunk%2Ftrac&old=7508&new_path=%2Ftrunk%2Ftrac&new=7500

FishEye has something similar:

http://svn.atlassian.com/fisheye/changelog/public/?cs=22609

However, it is only a comparison between the selected and previous revision.