Hrrm -- I would expect this to never work, because the Git and Mercurial pathways in DiffusionMergedCommitsQueryConduitAPIMethod end with:

return DiffusionQuery::loadHistoryForCommitIdentifiers(...)

..and it looks like that always returns objects (DiffusionPathChange objects). Then this will try to decode them, so I'd expect that it breaks the working case on its own.

In particular, it looks like this method does not work when called over HTTP currently (e.g., callsign=XPRF, commit=0a7a5f69f5d7 in the console on this install), so I think DiffusionMergedCommitsQueryConduitAPIMethod needs some return mpull($objects, 'toDictionary'); sort of stuff? Or am I crazy?