improve json when showStart' is given only a key

Before, the json contained file:key; change that to key:

If a file and a key are given, inclue both file: and key:

Messages.hs

@@ -54,12 +54,12 @@ import Types.Key
 import qualified Annex
 
 showStart :: String -> FilePath -> Annex ()
-showStart command file = outputMessage (JSON.start command $ Just file) $
+showStart command file = outputMessage (JSON.start command (Just file) Nothing) $
 	command ++ " " ++ file ++ " "
 
 showStart' :: String -> Key -> Maybe FilePath -> Annex ()
-showStart' command key afile = showStart command $
-	fromMaybe (key2file key) afile
+showStart' command key afile = outputMessage (JSON.start command afile (Just key)) $
+	command ++ " " ++ fromMaybe (key2file key) afile ++ " "
 
 showNote :: String -> Annex ()
 showNote s = outputMessage (JSON.note s) $ "(" ++ s ++ ") "
@@ -166,7 +166,7 @@ showFullJSON v = withOutputType $ liftIO . go
  -}
 showCustom :: String -> Annex Bool -> Annex ()
 showCustom command a = do
-	outputMessage (JSON.start command Nothing) ""
+	outputMessage (JSON.start command Nothing Nothing) ""
 	r <- a
 	outputMessage (JSON.end r) ""
 

Messages/JSON.hs

@@ -17,13 +17,18 @@ module Messages.JSON (
 import Text.JSON
 
 import qualified Utility.JSONStream as Stream
+import Types.Key
+import Data.Maybe
 
-start :: String -> Maybe String -> IO ()
-start command file =
-	putStr $ Stream.start $ ("command", command) : filepart file
+start :: String -> Maybe FilePath -> Maybe Key -> IO ()
+start command file key = putStr $ Stream.start $ catMaybes
+	[ part "command" (Just command)
+	, part "file" file
+	, part "key" (fmap key2file key)
+	]
   where
-	filepart Nothing = []
-	filepart (Just f) = [("file", f)]
+	part _ Nothing = Nothing
+	part l (Just v) = Just (l, v)
 
 end :: Bool -> IO ()
 end b = putStr $ Stream.add [("success", b)] ++ Stream.end