added push notifier thread, currently a no-op

Hooked up everything that needs to notify on pushes. Note that
syncNewRemote does not notify. This is probably ok, and I'd need to thread
more state through to make it do so.

This is only set up to support a single push notification method; I didn't
use a NotificationBroadcaster. Partly because I don't yet know what info
about pushes needs to be communicated, so my data types are only
preliminary.

Assistant/Pushes.hs

@@ -10,6 +10,7 @@ module Assistant.Pushes where
 import Common.Annex
 
 import Control.Concurrent.STM
+import Control.Concurrent.MSampleVar
 import Data.Time.Clock
 import qualified Data.Map as M
 
@@ -17,6 +18,9 @@ import qualified Data.Map as M
 type PushMap = M.Map Remote UTCTime
 type FailedPushMap = TMVar PushMap
 
+{- Used to notify about successful pushes. -}
+newtype PushNotifier = PushNotifier (MSampleVar ())
+
 {- The TMVar starts empty, and is left empty when there are no
  - failed pushes. This way we can block until there are some failed pushes.
  -}
@@ -44,3 +48,12 @@ changeFailedPushMap v a = atomically $
 		store m
 			| m == M.empty = noop
 			| otherwise = putTMVar v $! m
+
+newPushNotifier :: IO PushNotifier
+newPushNotifier = PushNotifier <$> newEmptySV
+
+notifyPush :: PushNotifier -> IO ()
+notifyPush (PushNotifier sv) = writeSV sv ()
+
+waitPush :: PushNotifier -> IO ()
+waitPush (PushNotifier sv) = readSV sv