According to the GNU manual, the format is:
<<<
GNU.sparse.size=size
GNU.sparse.numblocks=numblocks
repeat numblocks times
GNU.sparse.offset=offset
GNU.sparse.numbytes=numbytes
end repeat
>>>
The logic in parsePAX converts the repeating sequence of
(offset, numbytes) pairs (which is not PAX compliant) into a single
comma-delimited list of numbers (which is now PAX compliant).
Thus, we validate the following:
* The (offset, numbytes) headers must come in the correct order.
* The ',' delimiter cannot appear in the value.
We do not validate that the value is a parsible decimal since that
will be determined later.