copies-rust: split up combine_changeset_copies function into a struct … such that each iteration of its former loop is now a method call, with the caller driving the loop. This entirely removes the need for the `DataHolder` hack: the method now takes a `ChangedFiles<'_>` parameter that borrows a bytes buffer that can be owned by the caller’s stack frame, just for the duration of that call. Differential Revision: https://phab.mercurial-scm.org/D9683