Use process-file instead of shell-command-to-string

[fourier]

Suggested by StefanM:

    (let* ((file1 (file-name-directory (expand-file-name file1)))
           (file2 (file-name-directory (expand-file-name file2)))
           (default-directory (file-name-directory file1))
           (file1 (file-relative-name file1))
           (file2 (file-relative-name file2))
           (exit-code
            (process-file diff-command nil nil nil
                          "-q" file1 file2)))
      (if (and (numberp exit-code) (= exit-code 0))
          'same 'diff))

[fourier]

Fixed