Clarify test purpose.

Change-Id: I94d56689613114a012579610f31e078577ba7c43
diff --git a/fs/gitilesfs_test.go b/fs/gitilesfs_test.go
index f29c5e9..88a826c 100644
--- a/fs/gitilesfs_test.go
+++ b/fs/gitilesfs_test.go
@@ -220,7 +220,8 @@
 	}
 	defer fix.cleanup()
 
-	// Add a git repo; this doesn't have the requested blob.
+	// Add a git repo; this doesn't have the requested blob, but
+	// we can still get it from our (fake) HTTP gitiles server.
 	cmd := exec.Command("/bin/sh", "-c",
 		strings.Join([]string{
 			"mkdir -p localhost/platform/build/kati.git",