Don't clobber 'comint-input-autoexpand' in 'read-shell-command'

* lisp/shell.el (shell-completion-vars): Set only the
buffer-local value of 'comint-input-autoexpand'.  (Bug#34815)
1 job for emacs-26 in 27 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1053
test

00:27:02