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>
When is this expected to be called and could the look up time complexity change be relevant?  Appears to only be a single call but I am not familiar enough with the code to understand the call hierarchy.