(autodetect-patch-directory-root) doesn't get confused by symlinks

Prior to this patch you could have a scenario where

- The filename in the first hunk refers to a real file
- The emacs buffer returned by (buffer-file-name) is a symlink to this file

Since (autodetect-patch-directory-root) comparse paths, this would make the
comparison fail in the wrong place, and (autodetect-patch-directory-root) would
return a wrong result