[1.5.x] Fixed #19423 -- Prevented ModelAdmin sharing widgets due to formfield_overrides

Thanks joebuyer at manycycles.com for the report and Simon Charette
for the review.
Backport of 04e6542b5 from master.

tests/regressiontests/admin_widgets/models.py

@@ -20,6 +20,7 @@ class Member(models.Model):
 @python_2_unicode_compatible
 class Band(models.Model):
     name = models.CharField(max_length=100)
+    style = models.CharField(max_length=20)
     members = models.ManyToManyField(Member)
 
     def __str__(self):