Make "read-tree" merge the trees it reads by giving them consecutive states.

Make "read-tree" merge the trees it reads by giving them consecutive states.

Normally you'd use state 0 for the "merged" state, and start out with
state 1 being "origin", state 2 being "first tree" and state 3 being
"second tree".

Once all the index entries are back in state 0, we have a successful
merge and can write the result tree back.