storage: filesystem, Ensure index is populated before use. Fixes go-g…

…it#148

storage/filesystem/object.go

@@ -545,6 +545,9 @@ func (s *ObjectStorage) HashesWithPrefix(prefix []byte) ([]plumbing.Hash, error)
 
 	// TODO: This could be faster with some idxfile changes,
 	// or diving into the packfile.
+	if err := s.requireIndex(); err != nil {
+		return nil, err
+	}
 	for _, index := range s.index {
 		ei, err := index.Entries()
 		if err != nil {