Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Mac/Tools/macfreeze/macfreezegui.py b/Mac/Tools/macfreeze/macfreezegui.py
index 7921a29..5dd3435 100644
--- a/Mac/Tools/macfreeze/macfreezegui.py
+++ b/Mac/Tools/macfreeze/macfreezegui.py
@@ -113,7 +113,7 @@
 	try:
 		debug = string.atoi(string.strip(debug))
 	except ValueError:
-		EasyDialogs.Message("Illegal debug value %s, set to zero."%`debug`)
+		EasyDialogs.Message("Illegal debug value %r, set to zero."%(debug,))
 		debug = 0
 	if gentype == ITEM_GENSOURCE:
 		return 'source', script, dirname, debug