bug#55149: 29.0.50; Commit f30625943e broke magit/with-editor

[Juri Linkov]

> Could you give some details about that?
> The `git-commit` bug involves a lot of code, so it would be very helpful
> to have a recipe that involves less code.

I have the same problem, and distilled it to the shortest test case:

```
(defun testa ()
  1)

(advice-add 'testa :after
            (lambda (&rest _) 2))

(defun testb ()
  (interactive)
  (message "! %S" (testa)))
```

In 28 it returns 1.
In 29 it returns 2.

The regression is that the return value of :after now overrides
the return value of the original function.