Same fixes for the purchase scope

1) Do not overwrite lines description if the line was not modified

2) Correctly recompute prices/sellers on quantity change.

When no new line was added to the PO through the matrix, the
prices/sellers were not recomputed, even if the qty was modified for
some products.

closes odoo/odoo#78902

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>