When adding hardlinks to an archive, tarfile does not set the size
of each additional link to zero as specified by the tar format [1].

In addition, the current test case hardlinks is also broken.
Instead of testing that the size of a hardlink to a non-empty file
is 0, it tests that the size to a empty file is zero, which cannot
fail.

A patch against current svn trunk is attached and was tested with
'python -m test.regrtest  -v test_tarfile'


1. http://www.gnu.org/software/tar/manual/tar.html#SEC170