rose_arch: fix mtime+size file source record

A file source using `mtime+size` as the `update-check` does not get
stored uniquely. If a number of sources have the same mtimes and sizes,
they will not be archived. This change fixes the problem.
Merge pull request #1055 from matthewrmshin/fix-rose-arch-mtime-size-file-source

rose_arch: fix mtime+size file source record