Fix update-cache to compare the blob of a symlink against the link-target
and not the file it points to. Also ignore all permissions applied to
links.

Thanks to Greg for recognizing this while he added our list of symlinks
back to the udev repository.
Signed-off-by: NKay Sievers <kay.sievers@vrfy.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>