(create_root_interval): Initialize position to 0

(create_root_interval): Initialize position to 0
for a string.
(interval_start_pos): New function.
(find_interval): Handle string positions starting at 0.
(adjust_intervals_for_insertion): Likewise.
(adjust_intervals_for_deletion): Likewise.
(compare_string_intervals): Likewise.
(graft_intervals_into_buffer): Set `position' in reproduce_tree value.
(copy_intervals): Init `position' to 0.