switch from System.Cmd.Utils to System.Process

Test suite now passes with -threaded!

I traced back all the hangs with -threaded to System.Cmd.Utils. It seems
it's just crappy/unsafe/outdated, and should not be used. System.Process
seems to be the cool new thing, so converted all the code to use it
instead.

In the process, --debug stopped printing commands it runs. I may try to
bring that back later.

Note that even SafeSystem was switched to use System.Process. Since that
was a modified version of code from System.Cmd.Utils, it needed to be
converted too. I also got rid of nearly all calls to forkProcess,
and all calls to executeFile, which I'm also doubtful about working
well with -threaded.

Git/UpdateIndex.hs

@@ -17,7 +17,7 @@ module Git.UpdateIndex (
 	stageSymlink
 ) where
 
-import System.Cmd.Utils
+import System.Process
 
 import Common
 import Git
@@ -37,12 +37,13 @@ pureStreamer !s = \streamer -> streamer s
 {- Streams content into update-index from a list of Streamers. -}
 streamUpdateIndex :: Repo -> [Streamer] -> IO ()
 streamUpdateIndex repo as = do
-	(p, h) <- hPipeTo "git" (toCommand $ gitCommandLine params repo)
+	(Just h, _, _, p) <- createProcess (proc "git" ps) { std_in = CreatePipe }
 	fileEncoding h
 	forM_ as (stream h)
 	hClose h
-	forceSuccess p
+	forceSuccessProcess p "git" ps
 	where
+		ps = toCommand $ gitCommandLine params repo
 		params = map Param ["update-index", "-z", "--index-info"]
 		stream h a = a (streamer h)
 		streamer h s = do