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>
@vbatts  you're not using ByName, so it isn't really fixing the problem, right? Is the idea to introduce a new version that will sort ByName? I might be missing something?