Turn on pretty print for disag json output

Currently, disag results are output as unformatted JSON. It's about the only place where the GSON builder does not have the pretty print option turned on. Example disag output files are all on the order of ~200 KB unformatted and under ~500 KB after being formatted, so the files aren't too big.

mentioned in merge request !710 (merged)

mentioned in commit a1170b7c

closed with merge request !710 (merged)