diff: fix hardlink's target handling for unnormalized path

Commit 461ec875 ("diff: normalize path before considering the difference") normalized the paths used by diffing process. However, the paths are also used in various places inside diff.py, and I missed one case: hardlink handling. The code tried to make sure hardlink's target is included in the tarball before the hardlink itself, but it looked it up using the unnormalized path instead of normalized path so it thought the hardlink's target has not changed. So use normalized path instead.

Fixes: 461ec875 ("diff: normalize path before considering the difference") Fixes: #9 (closed)