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.

Assistant/Threads/Committer.hs

@@ -24,6 +24,7 @@ import Utility.ThreadScheduler
 import qualified Utility.Lsof as Lsof
 import qualified Utility.DirWatcher as DirWatcher
 import Types.KeySource
+import Types.Command
 import Config
 import Annex.Content
 import Annex.Ingest
@@ -286,7 +287,7 @@ handleAdds lockdowndir havelsof delayadd cs = returnWhen (null incomplete) $ do
 	  	ks = keySource ld
 		doadd = sanitycheck ks $ do
 			(mkey, _mcache) <- liftAnnex $ do
-				showStart "add" $ keyFilename ks
+				showStart "add" (keyFilename ks) (SeekInput [])
 				ingest nullMeterUpdate (Just $ LockedDown lockdownconfig ks) Nothing
 			maybe (failedingest change) (done change $ fromRawFilePath $ keyFilename ks) mkey
 	add _ _ = return Nothing