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/FuzzTest.hs

@@ -36,7 +36,7 @@ start :: CommandStart
 start = do
 	guardTest
 	logf <- fromRepo gitAnnexFuzzTestLogFile
-	showStart "fuzztest" (toRawFilePath logf)
+	showStart "fuzztest" (toRawFilePath logf) (SeekInput [])
 	logh <- liftIO $ openFile logf WriteMode
 	void $ forever $ fuzz logh
 	stop