This page doesn't mention what to do with new files that are added to the project (e.g test cases) Apparently you should do 'svn add' on them so that the get noticed by svn-create-patch. Also, although it mentions 'Make sure that your patch includes a ChangeLog entry' it doesn't mention using 'prepare-ChangeLog'.
prepare-ChangeLog is now mentioned. The 'svn add' issue still stands.
Fixed by r20354.