ensure that git-annex branch is pushed after a successful transfer

I now have this topology working:

   assistant ---> {bare repo, special remote} <--- assistant

And, I think, also this one:

        +----------- bare repo --------+
        v                              v
  assistant ---> special remote <--- assistant

While before with assistant <---> assistant connections, both sides got
location info updated after a transfer, in this topology, the bare repo
*might* get its location info updated, but the other assistant has no way to
know that it did. And a special remote doesn't record location info,
so transfers to it won't propigate out location log changes at all.

So, for these to work, after a transfer succeeds, the git-annex branch
needs to be pushed. This is done by recording a synthetic commit has
occurred, which lets the pusher handle pushing out the change (which will
include actually committing any still journalled changes to the git-annex
branch).

Of course, this means rather a lot more syncing action than happened
before. At least the pusher bundles together very close together pushes,
somewhat. Currently it just waits 2 seconds between each push.

Command/WebApp.hs

@@ -16,6 +16,7 @@ import Assistant.ScanRemotes
 import Assistant.TransferQueue
 import Assistant.TransferSlots
 import Assistant.Pushes
+import Assistant.Commits
 import Assistant.Threads.WebApp
 import Assistant.WebApp
 import Assistant.Install
@@ -106,11 +107,13 @@ firstRun = do
 	transferslots <- newTransferSlots
 	urlrenderer <- newUrlRenderer
 	pushnotifier <- newPushNotifier
+	commitchan <- newCommitChan
 	v <- newEmptyMVar
 	let callback a = Just $ a v
 	void $ runNamedThread dstatus $
 		webAppThread Nothing dstatus scanremotes
-			transferqueue transferslots pushnotifier urlrenderer
+			transferqueue transferslots pushnotifier commitchan
+			urlrenderer
 			(callback signaler) (callback mainthread)
 	where
 		signaler v = do