refs/debug: quote prefix

This makes the empty prefix ("") stand out better.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/debug.c

@@ -232,7 +232,8 @@ debug_ref_iterator_begin(struct ref_store *ref_store, const char *prefix,
 	struct debug_ref_iterator *diter = xcalloc(1, sizeof(*diter));
 	base_ref_iterator_init(&diter->base, &debug_ref_iterator_vtable, 1);
 	diter->iter = res;
-	trace_printf_key(&trace_refs, "ref_iterator_begin: %s (0x%x)\n", prefix, flags);
+	trace_printf_key(&trace_refs, "ref_iterator_begin: \"%s\" (0x%x)\n",
+			 prefix, flags);
 	return &diter->base;
 }