bug#48181: incremental autoloads update can put output in wrong file

[Glenn Morris <rgm@gnu.org>]

Package: emacs
Version: 28.0.50

I tried to figure out exactly why 68bf917896 caused non-bootstrap builds
to fail, because IIUC in theory it shouldn't have. I think this is why:

1) before 68bf917896, lisp/loaddefs.el contains sections like this:

  ;;;### (autoloads nil "texnfo-upd" "textmodes/texnfo-upd.el" (0 0
  ;;;;;;  0 0))
  ;;; Generated autoloads from textmodes/texnfo-upd.el

  (register-definition-prefixes "texnfo-upd" '("texinfo-"))

So even though there aren't any real autoloads in texnfo-upd.el, it's
not in the "no autoloads" section.

2) 68bf917896 added real autoload cookies to texnfo-upd.el, and set
generated-autoload-file to texinfo-loaddefs.el.

3) make-directory-autoloads correctly finds that texnfo-upd.el is newer
than loaddefs.el, so we execute this branch:
  
  (t
   (setq changed t)
   (autoload-remove-section (match-beginning 0))
   (if (autoload-generate-file-autoloads
         ;; Passing `current-buffer' makes it insert at point.   
         file (current-buffer) buffer-file-name)
        (push file no-autoloads)))

Because a non-nil OUTBUF is passed to autoload-generate-file-autoloads,
it ignores the buffer-local setting for generated-autoload-file,
and so puts the autoloads in the main loaddefs.el file, which is wrong.


In contrast, if the input loaddefs.el file in step 1 had texnfo-upd.el
in the "no loaddefs" section, then we would execute this branch:

  ;; Passing nil as second argument forces                             
  ;; autoload-generate-file-autoloads to look for the right            
  ;; spot where to insert each autoloads section.                      
  ((setq file-time
         (autoload-generate-file-autoloads file nil buffer-file-name))
 
As the comment says, this version has OUTBUF nil, so the loaddefs end up
the right file.