Fixed #10362 -- An update() that only affects a parent model no longer crashes.

This includes a fairly large refactor of the update() query path (and
the initial portions of constructing the SQL for any query). The
previous code appears to have been only working more or less by accident
and was very fragile.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9967 bcc190cf-cafb-0310-a4f2-bffc1f526a37