avoid space leak writing merge

This reduces the memory use of a merge by 1/3rd. The space leak was
apparently because the whole update-index input was generated strictly, not
lazily.

I wondered if the change to ByteStrings contributed to this, due to the
need to convert with L.pack here. But going back to the old code, I still
see a much similar leak, and worse performance besides due to it not using
ByteStrings.

The fix is to just hPutStr the lines repeatedly. (Note the \0 is written
separately, to avoid allocation overheads in adding it to the string.)
The Git.pipeWrite interface is probably just wrong for any large inputs to
git. This was the only place using it for input of any size.

There is still at least one other space leak in the merge code.

Git/UnionMerge.hs

@@ -46,10 +46,13 @@ merge_index h repo bs =
  - earlier ones, so the list can be generated from any combination of
  - ls_tree, merge_trees, and merge_tree_index. -}
 update_index :: Repo -> [String] -> IO ()
-update_index repo l = togit ["update-index", "-z", "--index-info"] (join "\0" l)
+update_index repo l = do
+	(p, h) <- hPipeTo "git" (toCommand $ Git.gitCommandLine params repo)
+	mapM_ (\s -> hPutStr h s >> hPutStr h "\0") l
+	hClose h
+	forceSuccess p
 	where
-		togit ps content = pipeWrite (map Param ps) (L.pack content) repo
-			>>= forceSuccess
+		params = map Param ["update-index", "-z", "--index-info"]
 
 {- Generates a line suitable to be fed into update-index, to add
  - a given file with a given sha. -}