Better manage memory usage of condensediffs.

condensediffs had some memory issues. Even though we were using
QuerySet.iterator, it seems memory was still increasing far too much.

To work around this, we're using a couple tricks that we used in
loaddb and dumpdb. We're only operating on batches of 200 diffs at a
time, and are resetting queries and garbage-collecting after each batch.

The query resets shouldn't impact production installs, since DEBUG
should be False, but it's a precaution. We're also forcing DEBUG to be
False in the management command as well.

With these changes, memory still increases over time, to a degree, but
seems to stabilize. This is with a sample set of over 8200 diffs.

Reviewed at https://reviews.reviewboard.org/r/5870/
Improve the output of condensediffs.

condensediffs will now show a percentage of completion, making it easier
to see how much longer the operation will take. It also formats the
resulting byte counts with commas, for readability.

Reviewed at https://reviews.reviewboard.org/r/5871/