On a non-existent directory get-free-disk-space returns nil, eg.

    (get-free-disk-space "/no/such/dir")
    => nil

But if it's done by a relative path from a default-directory which
doesn't exist, then the answer is the space in your home directory

    (let ((default-directory "/no/such/dir"))
      (get-free-disk-space "."))
    => 3705637

I hoped the two answers would be the same.

This is with the "df"-running code in get-free-disk-space, ie. no
`file-system-info' function.

Perhaps absolutize per below with an inserted setq, the diff being just
re-indent.

(I still think the use of the home dir there is either complicated and
unnecessary, or complicated and necessary everywhere.  Or maybe better
call-process could not attempt a chdir at all if a caller doesn't care
about the current directory.  But leaving that unchanged as yet.)

2009-07-24  Kevin Ryde  <user42@zip.com.au>

	* files.el (get-free-disk-space): Absolutize the dir before
	changing default-directory, so as not to answer about a different
	directory.