avoid duplication of "stringsource" entries in filename table

--HG--
extra : amend_source : c0a1e78dbfd443585a80bd646b763617eeb6b7c5

Cython/Compiler/Code.py

@@ -1317,13 +1317,13 @@ class GlobalState(object):
     # File name state
     #
 
-    def lookup_filename(self, filename):
+    def lookup_filename(self, source_desc):
         try:
-            index = self.filename_table[filename]
+            index = self.filename_table[source_desc.get_filenametable_entry()]
         except KeyError:
             index = len(self.filename_list)
-            self.filename_list.append(filename)
-            self.filename_table[filename] = index
+            self.filename_list.append(source_desc)
+            self.filename_table[source_desc.get_filenametable_entry()] = index
         return index
 
     def commented_file_contents(self, source_desc):