Optimised union merging; now only runs git cat-file once.

Annex/Branch.hs

@@ -149,7 +149,8 @@ update = onceonly $ do
 			 - documentation advises users not to directly
 			 - modify the branch.
 			 -}
-			inRepo $ \g -> Git.UnionMerge.merge_index g branches
+			h <- catFileHandle
+			inRepo $ \g -> Git.UnionMerge.merge_index h g branches
 		ff <- if dirty then return False else tryFastForwardTo refs
 		unless ff $ inRepo $
 			Git.commit merge_desc fullname (nub $ fullname:refs)
@@ -280,7 +281,7 @@ get' staleok file = fromcache =<< getCache file
 		fromjournal Nothing
 			| staleok = withIndex frombranch
 			| otherwise = withIndexUpdate $ frombranch >>= cache
-		frombranch = catFile fullname file
+		frombranch = L.unpack <$> catFile fullname file
 		cache content = do
 			setCache file content
 			return content

Annex/CatFile.hs

@@ -6,18 +6,25 @@
  -}
 
 module Annex.CatFile (
-	catFile
+	catFile,
+	catFileHandle
 ) where
 
+import qualified Data.ByteString.Lazy.Char8 as L
+
 import Common.Annex
 import qualified Git.CatFile
 import qualified Annex
 
-catFile :: String -> FilePath -> Annex String
-catFile branch file = maybe startup go =<< Annex.getState Annex.catfilehandle
+catFile :: String -> FilePath -> Annex L.ByteString
+catFile branch file = do
+	h <- catFileHandle
+	liftIO $ Git.CatFile.catFile h branch file
+
+catFileHandle :: Annex Git.CatFile.CatFileHandle
+catFileHandle = maybe startup return =<< Annex.getState Annex.catfilehandle
 	where
 		startup = do
 			h <- inRepo Git.CatFile.catFileStart
 			Annex.changeState $ \s -> s { Annex.catfilehandle = Just h }
-			go h
-		go h = liftIO $ Git.CatFile.catFile h branch file
+			return h