Prevent caching of filediffs when condensing diffs.

Some users reported running out of memory when attempting to condense
diffs on large databases. This is caused by Django's default behavior
of caching retrieved objects when evaluating a queryset. It'd end up
caching all objects it'd fetch, eating up all memory.

Using iterator() bypasses this, preventing caching and giving us the
objects directly. It should prevent the memory issues people were
hitting.