DiffRepository should not insert new resources at the end of the repository

DiffRepository should not insert new resources at the end of the repository. When merging a patch, it makes it extremely likely that resources added by two development branches will conflict.

DiffRepository should instead add resources somewhere in the middle at a deterministic place. If the repository is sorted, it should merge into alphabetical order by test case name. If it is not sorted, or only somewhat sorted, it should place the new resource into approximately the right position: resource "aardvark" will go towards the start, and resource "zebra" will go near the end.