start one git-cat-file per index file

This reverts 1c83b6c439 and properly fixes
the issue discussed there.

This makes git-annex behave much nicer in direct mode.

Annex/CatFile.hs

@@ -1,6 +1,6 @@
 {- git cat-file interface, with handle automatically stored in the Annex monad
  -
- - Copyright 2011 Joey Hess <joey@kitenet.net>
+ - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -15,6 +15,7 @@ module Annex.CatFile (
 ) where
 
 import qualified Data.ByteString.Lazy as L
+import qualified Data.Map as M
 
 import Common.Annex
 import qualified Git
@@ -37,13 +38,20 @@ catObjectDetails ref = do
 	h <- catFileHandle
 	liftIO $ Git.CatFile.catObjectDetails h ref
 
+{- There can be multiple index files, and a different cat-file is needed
+ - for each. This is selected by setting GIT_INDEX_FILE in the gitEnv. -}
 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 }
-		return h
+catFileHandle = do
+	m <- Annex.getState Annex.catfilehandles
+	indexfile <- fromMaybe "" . maybe Nothing (lookup "GIT_INDEX_FILE")
+		<$> fromRepo gitEnv
+	case M.lookup indexfile m of
+		Just h -> return h
+		Nothing -> do
+			h <- inRepo Git.CatFile.catFileStart
+			let m' = M.insert indexfile h m
+			Annex.changeState $ \s -> s { Annex.catfilehandles = m' }
+			return h
 
 {- From the Sha or Ref of a symlink back to the key. -}
 catKey :: Ref -> Annex (Maybe Key)
@@ -56,7 +64,7 @@ catKey ref = do
 {- From a file in git back to the key.
  -
  - Prefixing the file with ./ makes this work even if in a subdirectory
- - of a repo. For some reason, HEAD is sometimes needed.
+ - of a repo.
  -}
 catKeyFile :: FilePath -> Annex (Maybe Key)
-catKeyFile f = catKey $ Ref $ "HEAD:./" ++ f
+catKeyFile f = catKey $ Ref $ ":./" ++ f