[FIX] account_anglo_saxon: avoid duplication of price difference lines

When computing the price difference lines, in move_line_get of account_anglo_saxon, we loop on the result of super call for each lines (n * n times) to compute the price difference.
The product_id was used to match the returned line and the original invoice line. This was wrong as we could get several lines with the same product_id (and then get n * n price difference lines).
This patch adds the line id to the result of move_line_get (from account) so that account_anglo_saxon can filter more efficiently and only get one price difference per invoice line.
Fixes #704