bug#35171: 26.1; file-locked-p does not respect file-name-coding-system

[Olli Helenius]

The ‘file-locked-p’ function does not call ‘ENCODE_FILE’ on its argument
and thus can end up looking for a different file name than that which
was generated by ‘lock-buffer’.  Below is an Elisp expression that
demonstrates the issue.

;; Returns string "file-locked-p buggy" if the result of file-locked-p
;; is `nil' even when the buffer should be locked.
(let ((file-name-coding-system 'latin-1-unix)
      (create-lockfiles t))
  (let ((file (make-temp-file "file-locked-p-éö-" nil ".txt" "test"))
        (result nil))
    (unwind-protect
        (progn
          (find-file-existing file)
          (insert "modification")
          (lock-buffer)
          (if (eq nil (file-locked-p file))
              (setq result "file-locked-p buggy")
            (setq result "file-locked-p ok"))
          (save-buffer)
          (kill-buffer)
          result)
      (delete-file file nil))))

See also:
  
http://git.savannah.gnu.org/cgit/emacs.git/tree/src/filelock.c?h=emacs-26.1#n680
  
http://git.savannah.gnu.org/cgit/emacs.git/tree/src/filelock.c?h=emacs-26.1#n829

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-04-05 built on localhost
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Configured using:
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Important settings:
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/nix/store/553k4fjrf5d523zrjvm7jmw6zx23cbmf-emacs-packages-deps/share/emacs/site-lisp:
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Major mode: Lisp Interaction

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Features:
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Memory information:
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