When I generate a diff in some directory that I would afterwards delete,
the diff buffer appears to remain in the now non-existent directory
(according to `default-directory'), even if I want to generate a diff
for some other file.  The issue now is that make-process complains that
it cannot set the CWD when starting "diff", and instead fails with a
slightly confusing error message:

  start-process: Setting current directory: No such file or directory, /some/directory/that/doesnt/exist/anymore

This would fix the issue, but I don't think the solution is ideal: