diffs between a buffer and the underlying file

[Urs Thuermann <urs@isnogud.escape.de>]

I sometimes get into the following situation:  I open a file, make
some changes, then I want to see the changes before saving the file.
The best way to do this seems to be to set the region to the whole
buffer, write the region to some temporary file, and run diff on the
two files.  But I find this very unconvenient.

I'd like to have a function in emacs which can be applied to a buffer,
say buffer A, that opens a new buffer displaying all the changes
between the buffer A and the disk file, opened in buffer A.

Searching the emacs documentation I haven't found anything like this.
Only functions for diff'ing two buffers or two files.

urs