mtree: Don't fail verification for parents of a contents= filename

The files listed in the manifest as contents= are part of the files that are physically distributed, but they are not part of the logical hierarchy described by the manifest, so it wouldn't make sense to list their parents as part of the manifest. This will matter if we want to split files into subdirectories by a prefix of their hash, to avoid having a very large number of content-addressed files at top level.

steamrt/tasks#422