allow using Aeson for streaming JSON output

Keeping Text.JSON use for now, because it seems a better fit for most of
the commands, which don't use very structured JSON objects, but just output
whatever fields suites them. But this lets Aeson be used when a more
structured data type is available to serialize to JSON.

Command/Find.hs

@@ -66,7 +66,7 @@ start o file key = ifM (limited <||> inAnnex key)
 
 showFormatted :: Maybe Utility.Format.Format -> String -> [(String, String)] -> Annex ()
 showFormatted format unformatted vars =
-	unlessM (showFullJSON vars) $
+	unlessM (showFullJSON $ JSONObject vars) $
 		case format of
 			Nothing -> liftIO $ putStrLn unformatted
 			Just formatter -> liftIO $ putStr $