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.

Command/Config.hs

@@ -72,7 +72,7 @@ seek (UnsetConfig ck@(ConfigKey name)) = checkIsGlobalConfig ck $ commandAction
   where
 	ai = ActionItemOther (Just "unset")
 	si = SeekInput [decodeBS' name]
-seek (GetConfig ck@(ConfigKey name)) = checkIsGlobalConfig ck $ commandAction $
+seek (GetConfig ck) = checkIsGlobalConfig ck $ commandAction $
 	startingCustomOutput ai $ do
 		getGlobalConfig ck >>= \case
 			Just (ConfigValue v) -> liftIO $ S8.putStrLn v