finish CommandStart transition

The hoped for optimisation of CommandStart with -J did not materialize.
In fact, not runnign CommandStart in parallel is slower than -J3.
So, CommandStart are still run in parallel.

(The actual bad performance I've been seeing with -J in my big repo
has to do with building the remoteList.)

But, this is still progress toward making -J faster, because it gets rid
of the onlyActionOn roadblock in the way of making CommandCleanup jobs
run separate from CommandPerform jobs.

Added OnlyActionOn constructor for ActionItem which fixes the
onlyActionOn breakage in the last commit.

Made CustomOutput include an ActionItem, so even things using it can
specify OnlyActionOn.

In Command.Move and Command.Sync, there were CommandStarts that used
includeCommandAction, so output messages, which is no longer allowed.
Fixed by using startingCustomOutput, but that's still not quite right,
since it prevents message display for the includeCommandAction run
inside it too.

Command/Sync.hs

@@ -626,10 +626,14 @@ seekSyncContent o rs currbranch = do
 	
 	gokey mvar bloom (k, _) = go (Left bloom) mvar (AssociatedFile Nothing) k
 
-	go ebloom mvar af k = commandAction $ do
-		whenM (syncFile ebloom rs af k) $
-			void $ liftIO $ tryPutMVar mvar ()
-		return Nothing
+	go ebloom mvar af k = do
+		-- Run syncFile as a command action so file transfers run
+		-- concurrently.
+		let ai = OnlyActionOn k (ActionItemKey k)
+		commandAction $ startingCustomOutput ai $ do
+			whenM (syncFile ebloom rs af k) $
+				void $ liftIO $ tryPutMVar mvar ()
+			next $ return True
 
 {- If it's preferred content, and we don't have it, get it from one of the
  - listed remotes (preferring the cheaper earlier ones).
@@ -645,7 +649,7 @@ seekSyncContent o rs currbranch = do
  - Returns True if any file transfers were made.
  -}
 syncFile :: Either (Maybe (Bloom Key)) (Key -> Annex ()) -> [Remote] -> AssociatedFile -> Key -> Annex Bool
-syncFile ebloom rs af k = onlyActionOn' k $ do
+syncFile ebloom rs af k = do
 	inhere <- inAnnex k
 	locs <- map Remote.uuid <$> Remote.keyPossibilities k
 	let (have, lack) = partition (\r -> Remote.uuid r `elem` locs) rs