Re: grep-mode should print default-directory (again)

[Richard Stallman]

    AFAICS compilation-directory is just used with recompile and it gets set by
    M-x compile (through default-directory).

Damn, you're right.  That means the change to set compilation-directory
instead of default-directory was no good.  It broke the feature.

I can see only two ways to fix this:

* Go back to setting and saving default-directory, as before, and mark
  it safe.  (I don't find the arguments against this entirely convincing.)

* Make compilation-directory affect next-error, overriding
  default-directory.