debug.el: Prevent re-entering the debugger for the same error

We can have several active `handler-bind`s that all want to invoke the
debugger, in which case we can have the following sequence:

- The more deeply nested handler calls the debugger.
- After a while the user invokes `debugger-continue`.
- `signal_or_quit` propagates the error up the stack to the
  second handler, which calls the debugger again.
- The user thus ends up right back at the same place, as if
  `debugger-continue` had not be processed.

Fix this by remembering the last processed error and skipping
the debugger if we bump into it again.

* lisp/emacs-lisp/debug.el (debugger--last-error): New var.
(debugger--duplicate-p): New function.
(debug): Use them.

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