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>
this is the loook up for the build cache of an upload of the build context.
Look-up time is pretty negligible here.