Allow ForeignKey to a parent class when using inlines

[sil]

At the moment, if you have a hierarchy:

class Place(models.Model):

name = models.CharField(max_length=50)
address = models.CharField(max_length=80)

class Restaurant(Place):

serves_hot_dogs = models.BooleanField()
serves_pizza = models.BooleanField()

(as per ​http://www.djangoproject.com/documentation/model-api/#multi-table-inheritance), it doesn't interact all that well with inlined ForeignKeyed models. Imagine you had:

class Owner:

name = models.CharField(max_length=100)
place = models.ForeignKey(Place)

(so a Place can have multiple Owners). This also works fine with Restaurants, because every Restaurant is-a Place. However, if you want to inline the Owners:

class OwnerInline(admin.TabularInline):

model = Owner
extra = 5

class PlaceAdmin(admin.ModelAdmin):

model = Place
inlines = [OwnerInline]

then this won't work; if you try and create a Restaurant, you get a complaint that Owner has no ForeignKey to Restaurant (because technically it doesn't). This is because (new)forms._get_foreign_key checks that the ForeignKey points to this model itself, where it should really be checking whether the ForeignKey points to this model or any of its ancestor classes. The attached patch rectifies this.