handover: for intra-cell HO, use RR Assignment Command, not RR Handover Command
Handover Decision algorithms may decide to do a re-assignment within the same cell.
In practical tests, some time ago, I noticed that using the RR Assignment Command wasn't working here.
Hence we currently send an RR Handover Command referencing the same cell, which is kind of weird.
Now I saw one reason why Assignment doesn't work for handover: in bsc_api.c, in handle_ass_compl(),
we receive the RR Assignment Complete message and dispatch an SS_LCHAN signal S_LCHAN_ASSIGNMENT_COMPL.
However, throughout osmo-bsc, there is not a single handler for S_LCHAN_ASSIGNMENT_COMPL.
This might be another casualty from the split-off from osmo-nitb.
Anyway, it doesn't make sense to me that we send an S_LCHAN_ASSIGNMENT_COMPL,
while the remaining code triggers bsc_subscr_conn_fsm.c GSCON_EV_* events directly.
Fix: Actually handle RR Assignment Complete during handover,
and send RR Assignment Command instead of RR Handover Command for intra-cell HO.
This is not high priority, and should wait until after intra-BSC handover has rippled through the lchan allocation code.
The new FSM implementations have been merged to osmo-bsc master, but we are still using an RR Handover Command to re-assign an lchan within the same cell; it works ok, but an RR Assignment Command would make more sense. The code is now ready to change this and use the Assignment FSM (?) or use RR Assignment Command from handover FSM (?) for handovers where source and target cells are identical.