[IMP] models: in recompute, do not store fields that should not be recomputed

Consider a new field that uses the same compute method as another existing
field.  When the field is introduced in database, its value must be computed on
existing records.  In such a case, the existing field should not be written, as
its value is not supposed to have changed.  Not writing on the existing field
can avoid useless recomputations in cascade, which is the reason we introduce
this patch.

openerp/models.py

@@ -5667,7 +5667,11 @@ class BaseModel(object):
         while self.env.has_todo():
             field, recs = self.env.get_todo()
             # evaluate the fields to recompute, and save them to database
-            names = [f.name for f in field.computed_fields if f.store]
+            names = [
+                f.name
+                for f in field.computed_fields
+                if f.store and self.env.field_todo(f)
+            ]
             for rec in recs:
                 try:
                     values = rec._convert_to_write({