Fix usage of timedelta.total_seconds() in condensediffs.

The `total_seconds()` method doesn't exist in Python 2.6, so we can't use it
yet. This change replaces it with the equivalent implementation (and moves us
over to using true division throughout this file as a side effect).

Testing done:
Condensed some diffs.

Reviewed at https://reviews.reviewboard.org/r/6304/
Merge branch 'release-2.0.x'