Fixed #1254: pdb fails to launch some script.

Lib/pdb.py

@@ -1123,7 +1123,7 @@ see no sign that the breakpoint was reached.
         # Start with fresh empty copy of globals and locals and tell the script
         # that it's being run as __main__ to avoid scripts being able to access
         # the pdb.py namespace.
-        globals_ = {"__name__" : "__main__"}
+        globals_ = {"__name__" : "__main__", "__file__" : filename}
         locals_ = globals_
 
         # When bdb sets tracing, a number of call and line events happens