[Cao, Minggui] it is a ring buffer, if the buffer is not full
filled and full, the index shall be Handled differently. For
example, if total buffer size 8, now just buffered 4, the index
shall Be just in 3-0;

Understood. But we don't need to check it.

For key_UP, it can do like:
If ( --p_shell->selected_index < 0 )
p_shell->selected_index += p_shell->total_buffered_cmds;

[Cao, Minggui] there is a rule for history cmds: it is not a loop, if press up or
down key always, it will go up/down to the first or last command as the end one,
so the user will know the history is to end.
1. if the ring buffer not full, up key will go until 0 index, then it will not
change any more.
2. if the ring buffer is full, up key will also go to an end index too and
will not change any more then.

I updated this patch to V3, and sent it out, combine "input_line" and
"buffered_cmds" as "buffered_line", and simplify some logic as your comments.
Could you have a review?