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.

Types/Command.hs

@@ -40,7 +40,7 @@ type CommandCleanup = Annex Bool
 
 {- Input that was seeked on to make an ActionItem. Eg, the input filename,
  - or directory name. -}
-newtype SeekInput = SeekInput [String]
+newtype SeekInput = SeekInput { fromSeekInput :: [String] }
 	deriving (Show)
 
 {- Message that is displayed when starting to perform an action on