problem :
  1 - balance transaction A is indexed at date t1, a quantity
      diff of 1 is found on line a, so -1 is inserted into stock
  2 - for some reasons, a movement with date t2 with t2 < t1
      was not well indexed, after a reindex it is now correctly
      indexed
  3 - we reindex the balance transaction, thanks to to 2) there
      no diff any more, but due to bug the diff of -1 is not
      removed from the stock.
- add test for validating the fix of this bug

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43307 20353a03-c40f-0410-a6d1-a30d3c3de9de