diff -r option to ignore missing files

[Bryce Nesbitt (mailing list account)]

This is a request for a new switch to diff -- "ignore files that don't 
exist" (e.g. suppress the  "Only in" message).

Why?

Sometimes I build a sparse tree with only the files I've changed, and I 
want to diff it to the original.

And, of course, symmetry with the other "ignore" switches.

            -Bryce