[FIX] account: tax sign

When changing manually the the amount of tax in supplier invoice,
the tax sign of the tax must be kept.

opw:645691

addons/account/account_invoice.py

@@ -1533,7 +1533,7 @@ class account_invoice_tax(models.Model):
             currency = self.env['res.currency'].browse(currency_id)
             currency = currency.with_context(date=date_invoice or fields.Date.context_today(self))
             amount = currency.compute(amount, company.currency_id, round=False)
-        tax_sign = self.env['account.tax'].search([('name', '=', self.name), ('company_id', '=', self.company_id.id)]).tax_sign
+        tax_sign = self.tax_amount/self.amount
         return {'value': {'tax_amount': amount * tax_sign}}
 
     @api.v8