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it's just crappy/unsafe/outdated, and should not be used.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.

Backend/SHA.hs

@@ -12,6 +12,7 @@ import qualified Annex
 import Types.Backend
 import Types.Key
 import Types.KeySource
+import System.Process
 
 import qualified Build.SysConfig as SysConfig
 import Data.Digest.Pure.SHA
@@ -53,14 +54,16 @@ shaN shasize file filesize = do
 	showAction "checksum"
 	case shaCommand shasize filesize of
 		Left sha -> liftIO $ sha <$> L.readFile file
-		Right command -> liftIO $ runcommand command
+		Right command -> liftIO $ parse command . lines <$>
+			readProcess command (toCommand [File file]) ""
 	where
-		runcommand command =
-			pOpen ReadFromPipe command (toCommand [File file]) $ \h -> do
-				sha <- fst . separate (== ' ') <$> hGetLine h
-				if null sha
-					then error $ command ++ " parse error"
-					else return sha
+		parse command [] = bad command
+		parse command (l:_)
+			| null sha = bad command
+			| otherwise = sha
+			where
+				sha = fst $ separate (== ' ') l
+		bad command = error $ command ++ " parse error"
 
 shaCommand :: SHASize -> Integer -> Either (L.ByteString -> String) String
 shaCommand shasize filesize