Fix groups/diffs.py

[adehnert]

Figure out what ​https://diswww.mit.edu/menelaus/asa-db/562 is about, and fix it.

[adehnert]

Fixed:

commit d7dcfee959ba7773ad14849ca1bea216565cc44b
Author: Alex Dehnert <adehnert@mit.edu>
Date:   Sun Dec 25 03:48:39 2011 -0500

    Make recognition_date a datetime field
    
    There are two motivations for this:
    * Having the time a group filled out the startup form is kinda useful, and
      the group object stores when the startup form is filled out in the
      recognition_date field, until the group is recognized. If that field
      is a datetime field, then that data becomes available.
    * Apparently reversion has a bug whereby it will store whatever is assigned
      to the field, and then validate it on the way out. If you assign a datetime
      to a normal date field, then this will cause a validation error when you load
      it from the database. Since we were storing a datetime when doing group startup,
      this was occasionally causing breakage. (See Trac #47.)