Teach "fsck" not to follow subproject links

Teach "fsck" not to follow subproject links

Since the subprojects don't necessarily even exist in the current tree,
much less in the current git repository (they are totally independent
repositories), we do not want to try to follow the chain from one git
repository to another through a gitlink.

This involves teaching fsck to ignore references to gitlink objects from
a tree and from the current index.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>