The Portal diff view wants `[before after]` pairs of the original data,
not the three-part output of `clojure.data/diff`. This fixes the
`qp.debug/debug>` output from the preprocessing middleware to emit
correct diffs.

Also, since the diffs are not useful when the query is converted between
legacy and pMBQL, this also attaches the *converted* input query to the
output query's Clojure metadata, and this converted value is used as the
basis for the diff.