dired-diff
dired-diff is an interactive autoloaded compiled Lisp function in `dired-aux.el
'.
(dired-diff FILE &optional SWITCHES)
Compare file at point with file FILE using `diff'.
If called interactively, prompt for FILE. If the file at point
has a backup file, use that as the default. If the file at point
is a backup file, use its original. If the mark is active
in Transient Mark mode, use the file at the mark as the default.
(That's the mark set by C-SPC, not by Dired's
M-x dired-mark command.)
FILE is the first file given to `diff'. The file at point
is the second file given to `diff'.
With prefix arg, prompt for second argument SWITCHES, which is
the string of command switches for the third argument of `diff'.