split more stuff out of Git.hs

Git/Index.hs

@@ -0,0 +1,24 @@
+{- git index file stuff
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.Index where
+
+import System.Posix.Env (setEnv, unsetEnv, getEnv)
+
+{- Forces git to use the specified index file.
+ -
+ - Returns an action that will reset back to the default
+ - index file. -}
+override :: FilePath -> IO (IO ())
+override index = do
+	res <- getEnv var
+	setEnv var index True
+	return $ reset res
+	where
+		var = "GIT_INDEX_FILE"
+		reset (Just v) = setEnv var v True
+		reset _ = unsetEnv var