Find and display the differences between OLD and NEW files.
When called interactively, read NEW, then OLD, using the
minibuffer. The default for NEW is the current buffer's file
name, and the default for OLD is a backup file for NEW, if one
exists. If NO-ASYNC is non-nil, call diff synchronously.
When called interactively with a prefix argument, prompt
interactively for diff switches. Otherwise, the switches
specified in `diff-switches' are passed to the diff command.
(fn OLD NEW &optional SWITCHES NO-ASYNC)