fsck-objects: refactor checking for connectivity

fsck-objects: refactor checking for connectivity

This separates the connectivity check into separate codepaths,
one for reachable objects and the other for unreachable ones,
while adding a lot of comments to explain what is going on.

When checking an unreachable object, unlike a reachable one, we
do not have to complain if it does not exist (we used to
complain about a missing blob even when the only thing that
references it is a tree that is dangling).  Also we do not have
to check and complain about objects that are referenced by an
unreachable object.

This makes the messages from fsck-objects a lot less noisy and
more useful.

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