convert Watcher thread to Assistant monad

This is a nice win; much less code runs in Annex, so other threads have
more chances to run concurrently.

I do notice that renaming a file has gone from 1 to 2 commits. I think this
is due to the above improvement letting the committer run more frequently,
so it commits the rm first.

Assistant.hs

@@ -215,7 +215,7 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 #ifdef WITH_XMPP
 			, assist $ pushNotifierThread st dstatus pushnotifier
 #endif
-			, watch $ watchThread st dstatus transferqueue changechan
+			, watch $ watchThread
 			]
 		liftIO waitForTermination