normal_finish_command() and normal_prepare() assume that any pending
operator needs to be finished after any subsequent key.
Set `finish_op = false` in nv_event() to indicate that the pending
operator shouldn't be finished in normal_execute().
This is how nv_visual() indicates that 'v' or 'V' in operator-pending
mode should not finish the current pending operator.
fixes#5398fixes#6166 (partially; mappings are still interrupted)