saner format for metadata --json

metadata --json output format has changed, adding a inner json object
named "fields" which contains only the fields and their values.

This should be easier to parse than the old format, which mixed up
metadata fields with other keys in the json object.

Any consumers of the old format will need to be updated.

This adds a dependency on unordered-containers for parsing MetaData
from JSON, but it's a free dependency; aeson pulls in that library.

Command/MetaData.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2014 Joey Hess <id@joeyh.name>
+ - Copyright 2014-2016 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -10,9 +10,13 @@ module Command.MetaData where
 import Command
 import Annex.MetaData
 import Logs.MetaData
+import Messages.JSON (ParsedJSON(..))
 
 import qualified Data.Set as S
+import qualified Data.Text as T
+import qualified Data.ByteString.Lazy.UTF8 as BU
 import Data.Time.Clock.POSIX
+import Data.Aeson
 
 cmd :: Command
 cmd = withGlobalOptions ([jsonOption] ++ annexedMatchingOptions) $ 
@@ -95,10 +99,38 @@ perform now o k = case getSet o of
 
 cleanup :: Key -> CommandCleanup
 cleanup k = do
-	l <- map unwrapmeta . fromMetaData <$> getCurrentMetaData k
-	maybeShowJSON (JSONObject l)
-	showLongNote $ unlines $ concatMap showmeta l
+	m <- getCurrentMetaData k
+	let Object o = toJSON (MetaDataFields m)
+	maybeShowJSON $ AesonObject o
+	showLongNote $ unlines $ concatMap showmeta $
+		map unwrapmeta (fromMetaData m)
 	return True
   where
 	unwrapmeta (f, v) = (fromMetaField f, map fromMetaValue (S.toList v))
 	showmeta (f, vs) = map ((f ++ "=") ++) vs
+
+-- Metadata serialized to JSON in the field named "fields" of
+-- a larger object.
+newtype MetaDataFields = MetaDataFields MetaData
+	deriving (Show)
+
+instance ToJSON MetaDataFields where
+	toJSON (MetaDataFields m) = object [ (fieldsField, toJSON m) ]
+
+instance FromJSON MetaDataFields where
+	parseJSON (Object v) = do
+		f <- v .: fieldsField
+		case f of
+			Nothing -> return (MetaDataFields emptyMetaData)
+			Just v' -> MetaDataFields <$> parseJSON v'
+	parseJSON _ = fail "expected an object"
+
+fieldsField :: T.Text
+fieldsField = T.pack "fields"
+
+parseJSONInput :: String -> Maybe (Either FilePath Key, MetaData)
+parseJSONInput i = do
+	v <- decode (BU.fromString i)
+	case parsedAdded v of
+		Nothing -> return (parsedKeyfile v, emptyMetaData)
+		Just (MetaDataFields m) -> return (parsedKeyfile v, m)