presence (and incorrectness) of unique=True on ManyToManyFields. This has never
worked and generates invalid SQL. Now it's raised as an explicit error during
validation. Thanks to clamothe for the patch.

Still needs a docs change to make this clear, but that goes to the docs
refactor branch.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8488 bcc190cf-cafb-0310-a4f2-bffc1f526a37