import-tars: support hard links

import-tars: support hard links

Previously, we simply treated hard links as if they were plain files
with size 0, ignoring the link type "1" and hence the link target.

What we should do instead, of course, is to use the link target to get
at the import mark for the contents, even if we cannot recreate the hard
link per se, as Git has no concept of hard links.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>