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.hs

@@ -69,7 +69,9 @@
  - Thread 18: ConfigMonitor
  - 	Triggered by changes to the git-annex branch, checks for changed
  - 	config files, and reloads configs.
- - Thread 19: WebApp
+ - Thread 19: PushNotifier
+ - 	Notifies other repositories of pushes, using out of band signaling.
+ - Thread 20: WebApp
  - 	Spawns more threads as necessary to handle clients.
  - 	Displays the DaemonStatus.
  -
@@ -100,6 +102,11 @@
  - ScanRemotes (STM TMVar)
  - 	Remotes that have been disconnected, and should be scanned
  - 	are indicated by writing to this TMVar.
+ - BranchChanged (STM SampleVar)
+ - 	Changes to the git-annex branch are indicated by updating this
+ - 	SampleVar.
+ - PushNotifier (STM SampleVar)
+ - 	After successful pushes, this SampleVar is updated.
  - UrlRenderer (MVar)
  - 	A Yesod route rendering function is stored here. This allows
  - 	things that need to render Yesod routes to block until the webapp
@@ -133,6 +140,7 @@ import Assistant.Threads.NetWatcher
 import Assistant.Threads.TransferScanner
 import Assistant.Threads.TransferPoller
 import Assistant.Threads.ConfigMonitor
+import Assistant.Threads.PushNotifier
 #ifdef WITH_WEBAPP
 import Assistant.WebApp
 import Assistant.Threads.WebApp
@@ -180,6 +188,7 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 			transferslots <- newTransferSlots
 			scanremotes <- newScanRemoteMap
 			branchhandle <- newBranchChangeHandle
+			pushnotifier <- newPushNotifier
 #ifdef WITH_WEBAPP
 			urlrenderer <- newUrlRenderer
 #endif
@@ -191,19 +200,20 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 				, assist $ pairListenerThread st dstatus scanremotes urlrenderer
 #endif
 #endif
-				, assist $ pushThread st dstatus commitchan pushmap
-				, assist $ pushRetryThread st dstatus pushmap
+				, assist $ pushThread st dstatus commitchan pushmap pushnotifier
+				, assist $ pushRetryThread st dstatus pushmap pushnotifier
 				, assist $ mergeThread st dstatus transferqueue branchhandle
 				, assist $ transferWatcherThread st dstatus transferqueue
 				, assist $ transferPollerThread st dstatus
 				, assist $ transfererThread st dstatus transferqueue transferslots
 				, assist $ daemonStatusThread st dstatus
 				, assist $ sanityCheckerThread st dstatus transferqueue changechan
-				, assist $ mountWatcherThread st dstatus scanremotes
-				, assist $ netWatcherThread st dstatus scanremotes
-				, assist $ netWatcherFallbackThread st dstatus scanremotes
+				, assist $ mountWatcherThread st dstatus scanremotes pushnotifier
+				, assist $ netWatcherThread st dstatus scanremotes pushnotifier
+				, assist $ netWatcherFallbackThread st dstatus scanremotes pushnotifier
 				, assist $ transferScannerThread st dstatus scanremotes transferqueue
 				, assist $ configMonitorThread st dstatus branchhandle commitchan
+				, assist $ pushNotifierThread pushnotifier
 				, watch $ watchThread st dstatus transferqueue changechan
 				]
 			waitForTermination