bug#62599: 30.0.50; Improve ibuffer-diff-with-file

[Mattias Engdegård]

Thank you -- I'm no buffer expert but see no obvious flaws.

`verilog-diff-file-with-buffer` in verilog-mode.el looks vaguely similar, down 
to the dodgy unwind-protect, but it has stronger compatibility requirements so 
it probably can't use more recent diff machinery.