unify handling of unusual GIT_INDEX_FILE relative path

This is probably a git bug that stuck in its interface.

Annex/GitOverlay.hs

@@ -19,10 +19,9 @@ import qualified Annex
 {- Runs an action using a different git index file. -}
 withIndexFile :: FilePath -> Annex a -> Annex a
 withIndexFile f a = do
-	-- Workaround http://thread.gmane.org/gmane.comp.version-control.git/294880
-	absf <- liftIO $ absPath f
+	f' <- inRepo $ indexEnvVal f
 	withAltRepo
-		(\g -> addGitEnv g indexEnv absf)
+		(\g -> addGitEnv g indexEnv f')
 		(\g g' -> g' { gitEnv = gitEnv g })
 		a