Extend isamb_merge read handler with new insert mode.

Extend isamb_merge read handler with new insert mode.

Extend isamb_merge read handler, read_item, with new insert mode of
value 2. Like 1 this means "insertion", but, unlike 1, forces an existing
key to be replaced even though the comparison is 0. Until now ISAMB
has assumed "same" key for value of 0.