Diff Mode: Can I apply a difference to a buffer of my choosing?

[Alan Mackenzie]

Emacs 21.3.

I have a file, cc-defs.el.diff, which I created with a "cvs diff"
operation on the Emacs CVS at savannah, like this:

    # cvs diff -c -r1.36 -r 1.37 cc-defs.el > cc-defs.el.diff

.  I have loaded cc-defs.el.diff into a buffer in Diff Mode.  I want to
apply one of its diffs to a buffer (coincidentally also called
cc-defs.el).  However, when I try C-c C-a (`diff-apply-hunk'), Emacs
loads a different version of cc-defs.el, and applies the patch to that.

Is there any convenient way of associating cc-defs.el.diff with the
buffer I want to change?  Neither C-h m nor the Emacs manual seems to
suggest a way.

-- 
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
(like "aa"), remove half of them (leaving, say, "a").