Fix loss of list element after insertion at the head
Fix loss of list element after insertion at the head

Fixes #34. Thanks @aslakhellesoy for reporting the bug.

The problem was that when choosing the next list element among several
list elements inserted at the same position, getNext() was using string
comparison to order the sibling elements, when it should have used
Lamport timestamp comparison.

automerge/automerge,automerge/automerge