bug#30407: dired-diff ignores backup-directory-alist

[積丹尼 Dan Jacobson]

dired-diff prompts with
Diff p with (default            !home!jidanni!org!debconf!p.~8~): 
~/org/debconf/ #WRONG
Diff p with (default ~/.backups/!home!jidanni!org!debconf!p.~8~): 
~/org/debconf/ #Correct
when in .emacs the user does
(setq
 auto-save-list-file-prefix "~/.backups/.saves-";less clutter in $HOME
 backup-directory-alist '((".*" . "~/.backups/"))
 backup-by-copying-when-linked t ;don't destroy other hard links
 backup-by-copying-when-mismatch t)

It correctly detects that we are already at version ~8~ in that faraway
directory, but then it forgets to add the prefix to the default.

It then prompts with a non-existent wrong file doomed to fail if the
user hits RET.

Also using M-n, M-p do not make the default that it shows available to
the user to edit.

In fact the total correct prompt should probably look like one of
Diff p with (default ~/.backups/!home!jidanni!org!debconf!p.~8~): ~/.backups/
Diff p with (default ~/.backups/!home!jidanni!org!debconf!p.~8~): 
~/.backups/!home!jidanni!org!debconf!p.~8~
Diff p with: ~/.backups/!home!jidanni!org!debconf!p.~8~
The idea being that the user can easily change
Diff p with: ~/.backups/!home!jidanni!org!debconf!p.~8~ to
Diff p with: ~/.backups/!home!jidanni!org!debconf!p.~7~ etc.

emacs-version "25.2.2" and I do recall I reported something similar
years ago...