[FIX] base: in upgrade wizard, commit transaction before reloading registry

Not committing the current transaction may block the deinstallation of a
module.  Specifically, if a module adds a field on model `ir.module.module`,
its deinstallation will block when trying to remove the column, because the
wizard's pending transaction made a query on the table `ir_module_module`.

openerp/addons/base/module/wizard/base_module_upgrade.py

@@ -81,8 +81,9 @@ class base_module_upgrade(osv.osv_memory):
                 raise UserError(_('Following modules are not installed or unknown: %s') % ('\n\n' + '\n'.join(unmet_packages)))
 
             ir_module.download(cr, uid, ids, context=context)
-            cr.commit() # save before re-creating cursor below
 
+        # terminate transaction before re-creating cursor below
+        cr.commit()
         openerp.api.Environment.reset()
         openerp.modules.registry.RegistryManager.new(cr.dbname, update_module=True)