Fix block insertion in replace_chunk

Fix block insertion in replace_chunk

The code in replace_chunk() may try to add a tail block at the end
of the tree if you call it on the EOF block and the tail block has
to be re-created. This breaks the assumption of the tree code that
the last block never changes, and after changing the semantics of
insert_into_tree(), it will even add the tail block at the wrong
place (at the beginning of the tree).

Solve it by re-creating a head block instead.