the syncer now pushes out changes to remotes, in parallel

Note that, since this always pushes branch synced/master to the remote, it
assumes that master has already gotten all the commits that are on the
remote merged in. Otherwise, fast-forward prevention may prevent the push.

That's probably ok, because the next stage is to automatically detect
incoming pushes and merge.

Utility/Parallel.hs

@@ -0,0 +1,20 @@
+{- parallel processes
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.Parallel where
+
+import Common
+
+{- Runs an action in parallel with a set of values.
+ - Returns values that caused the action to fail. -}
+inParallel :: (v -> IO ()) -> [v] -> IO [v]
+inParallel a v = do
+	pids <- mapM (forkProcess . a) v
+	statuses <- mapM (getProcessStatus True False) pids
+	return $ map fst $ filter failed $ zip v statuses
+	where
+		failed (_, status) = status /= Just (Exited ExitSuccess)