fix memory leak when staging the journal

The list of files had to be retained until the end so it could be deleted.
Also, a list of update-index lines was generated and only then fed into it.
Now everything streams in constant space.

Git/HashObject.hs

@@ -1,6 +1,6 @@
 {- git hash-object interface
  -
- - Copyright 2011 Joey Hess <joey@kitenet.net>
+ - Copyright 2011-2012 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -11,23 +11,32 @@ import Common
 import Git
 import Git.Command
 
-{- Injects a set of files into git, returning the shas of the objects
- - and an IO action to call once the the shas have been used. -}
-hashFiles :: [FilePath] -> Repo -> IO ([Sha], IO ())
-hashFiles paths repo = do
-	(pid, fromh, toh) <- hPipeBoth "git" $ toCommand $ git_hash_object repo
+type HashObjectHandle = (PipeHandle, Handle, Handle)
+
+{- Starts git hash-object and returns a handle.  -}
+hashObjectStart :: Repo -> IO HashObjectHandle
+hashObjectStart repo = do
+	r@(_, _, toh) <- hPipeBoth "git" $
+		toCommand $ gitCommandLine params repo
 	fileEncoding toh
-	_ <- forkProcess (feeder toh)
-	hClose toh
-	shas <- map Ref . lines <$> hGetContents fromh
-	return (shas, ender fromh pid)
+	return r
 	where
-		git_hash_object = gitCommandLine
-			[Param "hash-object", Param "-w", Param "--stdin-paths"]
-		feeder toh = do
-			hPutStr toh $ unlines paths
-			hClose toh
-			exitSuccess
-		ender fromh pid = do
-			hClose fromh
-			forceSuccess pid
+		params =
+			[ Param "hash-object"
+			, Param "-w"
+			, Param "--stdin-paths"
+			]
+
+{- Stops git hash-object. -}
+hashObjectStop :: HashObjectHandle -> IO ()
+hashObjectStop (pid, from, to) = do
+	hClose to
+	hClose from
+	forceSuccess pid
+
+{- Injects a file into git, returning the shas of the objects. -}
+hashFile :: HashObjectHandle -> FilePath -> IO Sha
+hashFile (_, from, to) file = do
+	hPutStrLn to file
+	hFlush to
+	Ref <$> hGetLine from