Preserve multi socket link order when copying nodes or adding a new
group input sockets by linking directly to multi inputs from the group
input node's extension socket.
This is done by also copying the `multi_input_socket_index` when
the new links are created by copying existing or temporary links.
Reviewed By: Hans Goudey
Differential Revision: http://developer.blender.org/D14535
03d39a04a3