[FIX] models: do less snapshots in onchange()

Create a snapshot after having processed a batch of fields, then use it
to determine the next batch to process.

For instance, assume that field A modifies fields B and C, and that
field C modifies fields D, E and F:
 - before: 6 snapshots (process A, snapshot, process B, snapshot,
   process C, snapshot, process D, snapshot, process E, snapshot,
   process F, snapshot)
 - after: 3 snapshots (process A, snapshot, process B, process C,
   snapshot, process D, process E, process F, snapshot)

odoo/models.py

@@ -5182,24 +5182,22 @@ def diff(self, other):
 
         result = {}
 
-        # process names in order (or the keys of values if no name given)
-        while todo:
-            name = todo.pop(0)
-            if name in done:
-                continue
-            done.add(name)
-
-            with env.do_in_onchange():
+        # process names in order
+        with env.do_in_onchange():
+            while todo:
                 # apply field-specific onchange methods
-                if field_onchange.get(name):
-                    record._onchange_eval(name, field_onchange[name], result)
+                for name in todo:
+                    if field_onchange.get(name):
+                        record._onchange_eval(name, field_onchange[name], result)
+                    done.add(name)
 
                 # make a snapshot (this forces evaluation of computed fields)
                 snapshot1 = Snapshot(record, nametree)
 
-                # determine which fields have been modified
+                # determine which fields to process for the next pass
+                todo.clear()
                 for name in nametree:
-                    if snapshot1[name] != snapshot0[name]:
+                    if name not in done and snapshot1[name] != snapshot0[name]:
                         todo.append(name)
 
         # determine values that have changed by comparing snapshots