concurrent-output, first pass

Output without -Jn should be unchanged from before. With -Jn,
concurrent-output is used for messages, but regions are not used yet, so
it's a mess.

Messages/Concurrent.hs

@@ -0,0 +1,33 @@
+{- git-annex concurrent output
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE CPP #-}
+
+module Messages.Concurrent where
+
+import Common.Annex
+import Messages.Internal
+import Types.Messages
+
+#ifdef WITH_CONCURRENTOUTPUT
+import qualified System.Console.Concurrent as Console
+#endif
+
+{- Enable concurrent output when that has been requested.
+ -
+ - This should only be run once per git-annex lifetime, with
+ - everything that might generate messages run inside it.
+ -}
+withConcurrentOutput :: Annex a -> Annex a
+#ifdef WITH_CONCURRENTOUTPUT
+withConcurrentOutput a = withOutputType go
+  where
+	go (ConcurrentOutput _) = Console.withConcurrentOutput a
+	go _ = a
+#else
+withConcurrentOutput = id
+#endif