Get history file from shell to debugger if it exists.

IPython/terminal/debugger.py

@@ -68,6 +68,8 b' class TerminalPdb(Pdb):'
                 self.debugger_history = FileHistory(os.path.expanduser(str(p)))
             else:
                 self.debugger_history = InMemoryHistory()
+        else:
+            self.debugger_history = self.shell.debugger_history
 
         options = dict(
             message=(lambda: PygmentsTokens(get_prompt_tokens())),