This can be used in order to count the number of added files by
diff.files().filter(..).len(), for example. If we add more advanced fileset
predicates, it can also be expressed as diff("added()").files().len(). The
latter would be cheaper to compute.
For #4154, I'll probably add DiffStats template type. It might be doable by
extending diff.files() method, but I don't think it's good idea to write stats
calculation logic in template language.
Closes#5272
fe54c6276e