bug#31704: 26.1; tramp-remote-path/shell: broken executable completion

[xristos <xristos@sdf.org>]

The issue lies in shell--command-completion-data which is called
by shell-command-completion, it only checks directories in
exec-path (local) rather than tramp-remote-path. This bug also
leaks the local exec-path to the remote server.

To reproduce:

emacs -Q

Evaluate:
(require 'tramp)

;; For debugging
(setq tramp-persistency-file-name nil
      tramp-verbose 10)

(add-to-list 'tramp-remote-path 'tramp-own-remote-path)

;; Assuming an account 'chris' exists at server 'remote',
;; to see the issue clearly make sure that there exists
;; a directory in the remote server PATH (e.g. ~/bin/)
;; but NOT in exec-path.

(let ((default-directory "/ssh:chris@remote:"))
  (shell "*remote*"))

;; Assuming executable 'testbin' exists in remote server ~/bin/
;; and not inside any directory in exec-path:

remote$ testb<TAB> ;; No completion

M-:
(add-to-list 'exec-path "~/bin/")

remote$ testb<TAB> ;; Completes fine

If one checks the tramp debug buffer, one will see tramp
checking (and leaking information to the remote server)
all the directories that exist in the local exec-path.