Use haskell setenv library to clean up several ugly workarounds for inability to manipulate the environment on windows.

Didn't know that this library existed!

This includes making git-annex not re-exec itself on start on windows, and
making the test suite on Windows run tests without forking.

Git/Index.hs

@@ -21,8 +21,8 @@ import Utility.Env
 override :: FilePath -> IO (IO ())
 override index = do
 	res <- getEnv var
-	void $ setEnv var index True
-	return $ void $ reset res
+	setEnv var index True
+	return $ reset res
   where
 	var = "GIT_INDEX_FILE"
 	reset (Just v) = setEnv var v True