shell: optimize history storage a bit

When padding is added to "fill" the gap in order to wrap to the
beginning of the ring buffer, such padding is attributed to the
current item being added. Let's attribute it to the previous entry
instead so it'll be freed along with that entry and make the space
available one entry sooner, increasing the chances for keeping the
current entry around longer.

If ring buffer is empty then always reset it up front to get best
alignment right away i.e. beginning of buffer.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>