Fix formatting bug in log line.

Change-Id: I27f9e85dcbab8850fe43197894ff197299a33ece

fs/gitilesfs.go

diff --git a/fs/gitilesfs.go b/fs/gitilesfs.go
index 72462c3..afc4261 100644
--- a/fs/gitilesfs.go
+++ b/fs/gitilesfs.go
@@ -190,7 +190,7 @@
 
 	f, err := r.fetchFile(id, clone)
 	if err != nil {
-		log.Printf("fetchFile(%s): %v", id, err)
+		log.Printf("fetchFile(%s): %v", id.String(), err)
 		return nil, syscall.ESPIPE
 	}