bugfix: restore index file env var

This fixes precommit, since in that hook, git sets the env var to write
to the lock file, which avoids git add failing due to the presence of the
lock file. (Took me a good hour and a half of confusion to figure this out.)

Test suite now passes 100%! Only the upgrade code still remains to be
written.

Branch.hs

@@ -68,16 +68,19 @@ genIndex g = do
 
 {- Runs an action using the branch's index file. -}
 withIndex :: Annex a -> Annex a
-withIndex a = do
+withIndex = withIndex' False
+withIndex' :: Bool -> Annex a -> Annex a
+withIndex' bootstrapping a = do
 	g <- Annex.gitRepo
 	let f = index g
-	liftIO $ Git.useIndex f
+	reset <- liftIO $ Git.useIndex f
 
-	e <- liftIO $ doesFileExist f
-	unless e $ liftIO $ genIndex g
+	unless bootstrapping $ do
+		e <- liftIO $ doesFileExist f
+		unless e $ liftIO $ genIndex g
 
 	r <- a
-	liftIO $ Git.useDefaultIndex
+	liftIO reset
 	return r
 
 withIndexUpdate :: Annex a -> Annex a
@@ -121,11 +124,8 @@ create = do
 		inorigin <- refexists origin
 		if inorigin
 			then liftIO $ Git.run g "branch" [Param name, Param origin]
-			else liftIO $ do
-				let f = index g
-				liftIO $ Git.useIndex f
-				GitUnionMerge.commit g "branch created" fullname []
-				liftIO $ Git.useDefaultIndex
+			else withIndex' True $
+				liftIO $ GitUnionMerge.commit g "branch created" fullname []
 	where
 		origin = "origin/" ++ name
 		refexists ref = do

GitRepo.hs

@@ -60,7 +60,6 @@ module GitRepo (
 	repoAbsPath,
 	reap,
 	useIndex,
-	useDefaultIndex,
 	hashObject,
 	getSha,
 	shaSize,
@@ -79,6 +78,7 @@ import System.Cmd.Utils
 import IO (bracket_)
 import Data.String.Utils
 import System.IO
+import IO (try)
 import qualified Data.Map as Map hiding (map, split)
 import Network.URI
 import Data.Maybe
@@ -88,7 +88,7 @@ import Codec.Binary.UTF8.String (encode)
 import Text.Printf
 import Data.List (isInfixOf, isPrefixOf, isSuffixOf)
 import System.Exit
-import System.Posix.Env (setEnv, unsetEnv)
+import System.Posix.Env (setEnv, unsetEnv, getEnv)
 
 import Utility
 
@@ -391,13 +391,18 @@ reap = do
 	r <- catch (getAnyProcessStatus False True) (\_ -> return Nothing)
 	maybe (return ()) (const reap) r
 
-{- Forces git to use the specified index file. -}
-useIndex :: FilePath -> IO ()
-useIndex index = setEnv "GIT_INDEX_FILE" index True
-
-{- Undoes useIndex -}
-useDefaultIndex :: IO ()
-useDefaultIndex = unsetEnv "GIT_INDEX_FILE"
+{- Forces git to use the specified index file.
+ - Returns an action that will reset back to the default
+ - index file. -}
+useIndex :: FilePath -> IO (IO ())
+useIndex index = do
+	res <- try $ getEnv var
+	setEnv var index True
+	return $ reset res
+	where
+		var = "GIT_INDEX_FILE"
+		reset (Right (Just v)) = setEnv var v True
+		reset _ = unsetEnv var
 
 {- Injects some content into git, returning its hash. -}
 hashObject :: Repo -> String -> IO String