gitindex/index.go: rename unused variables

Change-Id: Id2ce7598b6afd52c2455f5c3233103474e5a37d5
diff --git a/gitindex/index.go b/gitindex/index.go
index f805cc8..b2500cd 100644
--- a/gitindex/index.go
+++ b/gitindex/index.go
@@ -45,7 +45,7 @@
 	refDir := filepath.Join(dir, "refs")
 	if _, err := os.Lstat(refDir); err == nil {
 		if err := filepath.Walk(refDir,
-			func(name string, fi os.FileInfo, err error) error {
+			func(_ string, fi os.FileInfo, _ error) error {
 				if !fi.IsDir() && last.Before(fi.ModTime()) {
 					last = fi.ModTime()
 				}