Tentatively pull in changes for #2742 prefetch fix.

I believe the issue is that the state of `fks` / `backrefs` could be
incorrect if the loop finds relationships but does not exit (due to
failing the 2nd clause which checks the target_model).

peewee.py

@@ -7955,8 +7955,8 @@ def prefetch_add_subquery(sq, subqueries, prefetch_type):
            isinstance(subquery, ModelAlias):
             subquery = subquery.select()
         subquery_model = subquery.model
-        fks = backrefs = None
         for j in reversed(range(i + 1)):
+            fks = backrefs = None
             fixed = fixed_queries[j]
             last_query = fixed.query
             last_model = last_obj = fixed.model
@@ -7972,7 +7972,7 @@ def prefetch_add_subquery(sq, subqueries, prefetch_type):
                                       (target_model is None)):
                 break
 
-        if not fks and not backrefs:
+        else:
             tgt_err = ' using %s' % target_model if target_model else ''
             raise AttributeError('Error: unable to find foreign key for '
                                  'query: %s%s' % (subquery, tgt_err))