tarsum: name collision fix

If a tar were constructed with duplicate file names, then depending on
the order, it could result in same tarsum.

This does not break the existing checksums of correctly produced layers.

Signed-off-by: Vincent Batts <vbatts@redhat.com>
the point is that if there is an additional content in the tar archive, even if it were hidden under an updated path, that it would still render a different and consistent checksum. Let me add a couple more tests to double check this.