assert: fix _compare_eq_iterable: re-format both sides

[blueyed]

Follow-up to 946434c (pytest-dev#5924).

Before this patch the test would look like this:

{'env': {'sub...s wrapped'}}}} == {'env': {'sub...}}}, 'new': 1}
Omitting 1 identical items, use -vv to show
Right contains 1 more item:
{'new': 1}
Full diff:
  {
   'env': {'sub': {'long_a': 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa',
-                  'sub1': {'long_a': 'substring '
+                  'sub1': {'long_a': 'substring that gets wrapped'}}},
?                                                +++++++++++++++++ ++++
+  'new': 1,
-                                     'that '
-                                     'gets '
-                                     'wrapped'}}},
  }

Ref: pytest-dev#6137