sequencer: remove the 'arg' field from todo_item

sequencer: remove the 'arg' field from todo_item

The 'arg' field of todo_item used to store the address of the first byte
of the parameter of a command in a todo list.  It was associated with
the length of the parameter (the 'arg_len' field).

This replaces the 'arg' field by 'arg_offset'.  This new field does not
store the address of the parameter, but the position of the first
character of the parameter in the buffer.  todo_item_get_arg() is added
to return the address of the parameter of an item.

This will prevent todo_list_add_exec_commands() from having to do awful
pointer arithmetics when growing the todo list buffer.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>