Fix the file sizes and percentages shown for condensediffs.

When running condensediffs, the decoded diff content was being used for
the byte counts, rather than the stored base64 content. This gave an
inaccurate result for the total number of bytes and the bytes saved.

The percentage was also wrong, due to a bad calculation. Now it shows an
accurate savings as a percentage, and also shows up to 2 decimal places.

Reviewed at https://reviews.reviewboard.org/r/6208/