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

[M. Ian Graham]

> Oh, well, the recipe can even be simplyfied to defining just a
> do-nothing advice!

Ah ha. Return value shenanigans. Here's a no-server test case.

--- begin elisp ---
(advice-add 'original :after
            #'original--advice)

(defun original () "correct: original return value passes through")
(defun original--advice () "broken: advice function's return value is used")

(message (original))
--- end elisp ---

Output before f30625943e:
$ emacs -Q --script bug-recipe.el
correct: original return value passes through

Output after f30625943e:
$ emacs -Q --script bug-recipe.el
broken: advice function's return value is used

-Ian