select_related will cause table multiple times in FROM clause if table already in extra()

[rezzrovv]

If a QuerySet is built or initialized with select_related and extra() and extra contains a table that will ultimately be iterated over through the select_related, that table will not be picked up in fill_table_cache. The result is a query with the table twice in the FROM clause and results in an DB error when executed (at least on postgresql, can't speak for any other DBs). Patch submitted. Example error from postgresql:

ProgrammingError: 'ERROR: table name "client" specified more than once

[rezzrovv]

patch for query error

[Chris Beaven]

Issue looks like a valid bug, but that's a hackish patch

[Malcolm Tredinnick]

(In [6504]) queryset-refactor: Fixed handling of extra(tables=...). In passing, this solves
a duplicate table / bad SQL problem. Refs #2496.

[Malcolm Tredinnick]

(In [7477]) Merged the queryset-refactor branch into trunk.

This is a big internal change, but mostly backwards compatible with existing
code. Also adds a couple of new features.

Fixed #245, #1050, #1656, #1801, #2076, #2091, #2150, #2253, #2306, #2400, #2430, #2482, #2496, #2676, #2737, #2874, #2902, #2939, #3037, #3141, #3288, #3440, #3592, #3739, #4088, #4260, #4289, #4306, #4358, #4464, #4510, #4858, #5012, #5020, #5261, #5295, #5321, #5324, #5325, #5555, #5707, #5796, #5817, #5987, #6018, #6074, #6088, #6154, #6177, #6180, #6203, #6658