archive: ustar header checksum is computed unsigned

archive: ustar header checksum is computed unsigned

POSIX.1 (pax) is pretty clear on this:

  The chksum field shall be the ISO/IEC 646:1991 standard IRV
  representation of the octal value of the simple sum of all octets
  in the header logical record. Each octet in the header shall be
  treated as an unsigned value. These values shall be added to an
  unsigned integer, initialized to zero, the precision of which is
  not less than 17 bits. When calculating the checksum, the chksum
  field is treated as if it were all <space> characters.

so is GNU:

  http://www.gnu.org/software/tar/manual/html_node/Checksumming.html

Found by 7zip folks and reported by Rafał Mużyło.

Signed-off-by: Junio C Hamano <gitster@pobox.com>