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/List.hs

@@ -78,7 +78,7 @@ printHeader :: [(UUID, RemoteName, TrustLevel)] -> Annex ()
 printHeader l = liftIO $ putStrLn $ lheader $ map (\(_, n, t) -> (n, t)) l
 
 start :: [(UUID, RemoteName, TrustLevel)] -> SeekInput -> RawFilePath -> Key -> CommandStart
-start l si file key = do
+start l _si file key = do
 	ls <- S.fromList <$> keyLocations key
 	liftIO $ putStrLn $ format (map (\(u, _, t) -> (t, S.member u ls)) l) file
 	stop