add back debug logging

Make Utility.Process wrap the parts of System.Process that I use,
and add debug logging to them.

Also wrote some higher-level code that allows running an action
with handles to a processes stdin or stdout (or both), and checking
its exit status, all in a single function call.

As a bonus, the debug logging now indicates whether the process
is being run to read from it, feed it data, chat with it (writing and
reading), or just call it for its side effect.

Git/Queue.hs

@@ -19,7 +19,6 @@ module Git.Queue (
 
 import qualified Data.Map as M
 import System.IO
-import System.Process
 import Data.String.Utils
 
 import Utility.SafeCommand
@@ -148,13 +147,11 @@ runAction :: Repo -> Action -> IO ()
 runAction repo (UpdateIndexAction streamers) =
 	-- list is stored in reverse order
 	Git.UpdateIndex.streamUpdateIndex repo $ reverse streamers
-runAction repo action@(CommandAction {}) = do
-	(Just h, _, _, pid) <- createProcess (proc "xargs" params)
-		{ std_in = CreatePipe }
-	fileEncoding h
-	hPutStr h $ join "\0" $ getFiles action
-	hClose h
-	forceSuccessProcess pid "xargs" params
+runAction repo action@(CommandAction {}) =
+	withHandle StdinHandle createProcessSuccess (proc "xargs" params) $ \h -> do
+		fileEncoding h
+		hPutStr h $ join "\0" $ getFiles action
+		hClose h
 	where
 		params = "-0":"git":baseparams
 		baseparams = toCommand $ gitCommandLine