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>
@unclejack double check that you're testing commit d0c17170c2d2ce1576225f1e83271b5d778df003
I'm not seeing that error and there is no longer `sort.Strings(keys)`