join back threads before ending concurrent output so display works

I didn't really want to put allowConcurrentOutput in CmdLine.Action,
but there were dep loops and that was the best place available.

Command.hs

@@ -19,7 +19,6 @@ module Command (
 	whenAnnexed,
 	ifAnnexed,
 	isBareRepo,
-	allowConcurrentOutput,
 	module ReExported
 ) where
 
@@ -37,7 +36,6 @@ import CmdLine.Option as ReExported
 import CmdLine.GlobalSetter as ReExported
 import CmdLine.GitAnnex.Options as ReExported
 import Options.Applicative as ReExported hiding (command)
-import Messages.Internal (allowConcurrentOutput)
 
 import qualified Options.Applicative as O