Add connectivity tracking to fsck.

Add connectivity tracking to fsck.

This shows that I've lost track of one commit already. Most likely
because I forgot to update the .dircache/HEAD file when doing a
commit, so that the next commit referenced not the top-of-tree, but
the one older commit.

Having dangling commits is fine (in fact, you should always have
at least _one_ dangling commit in the top-of-tree). But it's
good to know about them.