Revert to old 23.1 logic of using the file at the mark as default.

Revert to old 23.1 logic of using the file at the mark as default.
* dired-aux.el (dired-diff): Use the file at the mark as default
if it's not the same as the current file, and the target dir is
the current dir or the mark is active.  Add the current file
as the arg of `dired-dwim-target-defaults'.  Use the default file
in the prompt.  (Bug#5225)