Make fsck-cache do better tree checking.

We check the ordering of the entries, and we verify that none
of the entries has a slash in it (this allows us to remove the
hacky "has_full_path" member from the tree structure, since we
now just test it by walking the tree entries instead).

tree.h

@@ -18,7 +18,6 @@ struct tree_entry_list {
 
 struct tree {
 	struct object object;
-	unsigned has_full_path : 1;
 	struct tree_entry_list *entries;
 };