add "input" field to json output

The use case of this field is mostly to support -J combined with --json.
When that is implemented, a user will be able to look at the field to
determine which of the requests they have sent it corresponds to.

The field typically has a single value in its list, but in some cases
mutliple values (eg 2 command-line params) are combined together and the
list will have more.

Note that json parsing was already non-strict, so old git-annex metadata
--json --batch can be fed json produced by the new git-annex and will
not stumble over the new field.

Messages/JSON.hs

@@ -1,6 +1,6 @@
 {- git-annex command-line JSON output and input
  -
- - Copyright 2011-2018 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -39,6 +39,7 @@ import Data.Monoid
 import Prelude
 
 import Types.Messages
+import Types.Command (SeekInput(..))
 import Key
 import Utility.Metered
 import Utility.Percentage
@@ -64,8 +65,8 @@ type JSONBuilder = Maybe (Object, Bool) -> Maybe (Object, Bool)
 none :: JSONBuilder
 none = id
 
-start :: String -> Maybe RawFilePath -> Maybe Key -> JSONBuilder
-start command file key _ = case j of
+start :: String -> Maybe RawFilePath -> Maybe Key -> SeekInput -> JSONBuilder
+start command file key si _ = case j of
 	Object o -> Just (o, False)
 	_ -> Nothing
   where
@@ -74,6 +75,7 @@ start command file key _ = case j of
 		, itemKey = key
 		, itemFile = fromRawFilePath <$> file
 		, itemAdded = Nothing
+		, itemSeekInput = si
 		}
 
 end :: Bool -> JSONBuilder
@@ -176,6 +178,7 @@ data JSONActionItem a = JSONActionItem
 	, itemKey :: Maybe Key
 	, itemFile :: Maybe FilePath
 	, itemAdded :: Maybe a -- for additional fields added by `add`
+	, itemSeekInput :: SeekInput
 	}
 	deriving (Show)
 
@@ -183,10 +186,11 @@ instance ToJSON' (JSONActionItem a) where
 	toJSON' i = object $ catMaybes
 		[ Just $ "command" .= itemCommand i
 		, case itemKey i of
-			Nothing -> Nothing
 			Just k -> Just $ "key" .= toJSON' k
+			Nothing -> Nothing
 		, Just $ "file" .= toJSON' (itemFile i)
 		-- itemAdded is not included; must be added later by 'add'
+		, Just $ "input" .= fromSeekInput (itemSeekInput i)
 		]
 
 instance FromJSON a => FromJSON (JSONActionItem a) where
@@ -195,6 +199,7 @@ instance FromJSON a => FromJSON (JSONActionItem a) where
 		<*> (maybe (return Nothing) parseJSON =<< (v .:? "key"))
 		<*> (v .:? "file")
 		<*> parseadded
+		<*> pure (SeekInput [])
 	  where
 		parseadded = (Just <$> parseJSON (Object v)) <|> return Nothing
 	parseJSON _ = mempty