Fix problem with auto-mode and dir-locals-collect-variables

Fix problem with auto-mode and dir-locals-collect-variables

* lisp/files.el (dir-locals-collect-variables): When run from
auto-mode, the file in question may not be an absolute path
name (bug#24016).

Example backtrace:

Debugger entered--Lisp error: (args-out-of-range "compile-1st-in-loa
  dir-locals-collect-variables(((emacs-lisp-mode (indent-tabs-mode))
  hack-dir-local-variables()
  hack-local-variables(no-mode)
  run-mode-hooks(diff-mode-hook)
  diff-mode()
  mm-display-inline-fontify((#<buffer  *mm*-923037> ("text/x-diff" (