diff (2.8.1) cannot work with really large files

[Darren Reed]

There is some brain dead logic in diff that seems to assume that
whichever files are being diff'd need to be read entirely into memory,
for which not even --large-spare-files fixes.

I have two 300MB text files and if I run diff on them, I get:
diff: memory exhausted

What poppycock.

I'm equally surprised that it doesn't appear to try mmap either.

Maybe we need:
--small-buffers-for-really-large-files

But I suspect that's not enough and that core parts of diff need
to be rewritten if it is going to assume that the entire file isn't in
memory.

Meanwhile, I think I'll write my own diff program that doesn't
need to allocate 600MB of RAM to tell me 5 lines are different.