Fix formatting error.

Change-Id: Ib08af9cb5079c788d36c6a59ba76f8646cbda946
diff --git a/fs/manifestfs_test.go b/fs/manifestfs_test.go
index 2f3aa86..6a2f41d 100644
--- a/fs/manifestfs_test.go
+++ b/fs/manifestfs_test.go
@@ -107,7 +107,7 @@
 
 	headID, err := git.NewOid(headSHA1)
 	if err != nil {
-		t.Fatal("NewOid(%q): %v", headSHA1, err)
+		t.Fatalf("NewOid(%q): %v", headSHA1, err)
 	}
 
 	tree, err := fix.cache.Tree.Get(headID)