[FIX] stock: programming error in average price computation for multiple lines of the same product

While processing a picking we must keep track of
previously processed lines as they modify the
stock on hand but are not yet included in the
`qty_available` function.
Negative stock on hand is handled as if
the stock was zero as far as the average
price computation is concerned.
[FIX] resolve conflict between
http://bazaar.launchpad.net/~ocb/ocb-addons/7.0/revision/9584
and
http://bazaar.launchpad.net/~openerp/openobject-addons/7.0/revision/9564