[FIX] core: prefetching of x2many lines in snapshot.diff()

The prefetching optimization was simply broken for x2many lines in the
snapshot method `diff()`.  Indeed, iterating on `line._prefetch_ids`
actually returns nothing, because this iterable object relies on the
value in cache of the x2many field, and the cache has been invalidated
before calling `diff()`.  In order to re-enable prefetching, we simply
populate the cache of x2many fields before looping on the lines.

Some testing with `account.move` and the relation `invoice_line_ids`:
For 335 invoice lines, changing the value of a field in the invoice
lines took about 1.9s with 1415 queries.  It now takes about 1.1s and 82
queries.

closes odoo/odoo#58687

X-original-commit: 5bd7ddd0850e56fbbae52b4478970331f9db1589
Signed-off-by: Raphael Collet (rco) <rco@openerp.com>
Co-authored-by: Raphael Collet <rco@odoo.com>

odoo/models.py

@@ -6023,6 +6023,14 @@ Fields:
                     else:
                         # x2many fields: serialize value as commands
                         result[name] = commands = [(5,)]
+                        # The purpose of the following line is to enable the prefetching.
+                        # In the loop below, line._prefetch_ids actually depends on the
+                        # value of record[name] in cache (see prefetch_ids on x2many
+                        # fields).  But the cache has been invalidated before calling
+                        # diff(), therefore evaluating line._prefetch_ids with an empty
+                        # cache simply returns nothing, which discards the prefetching
+                        # optimization!
+                        record[name]
                         for line_snapshot in self[name]:
                             line = line_snapshot['<record>']
                             line = line._origin or line