Optimise querying of checks in patch list view

Optimise querying of checks in patch list view

tl;dr: with about 300 mails from the patchwork list, according to
django-debug-toolbar, to render '/project/patchwork/list/'

Without this patch:
 - ~1.35 seconds of CPU time
 - 110 SQL queries, taking ~70ms

With this patch:
 - < 0.3 seconds of CPU time
 - 10 SQL queries, taking <20ms

How? Replace an .exclude() on a QuerySet with a list comprehension.
Yes, that's normally a pessimisation.  Surprisingly, it's an
optimisation here.  Why? Where we're looking at patches in anything
that uses a generic_list() in the view, we do a prefetch_related. But,
if we then do a .filter or a .exclude, that throws out the existing,
cached information, and does another query. (See the Django docs on
prefetch_related)

So, do it 'by hand' in Python instead.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephenfinucane@hotmail.com>