bug#7777: 24.0.50; incompatible change for `directory-abbrev-alist' not in NEWS

[Drew Adams]

> > That means that *Every* existing customization is broken 
> > (or at least contradicts the "should").
> 
> It isn't and it doesn't.  If there are no file names with embedded
> newlines, both "^" and "\`" do the same.

Yes, I know that.

> The "should" is to remind this issue to those who want their
> customizations DTRT with newlines in file names; previously,
> they needed to figure that by themselves, or learn it the hard way.

"Previously" there was already a similar "should" (for `^').
I have no problem with the "should", in any case.

> > This incompatible change should be mentioned in the NEWS.
> 
> There was no change, except in the doc string (and in the value
> suggested during customization).  No code changes, and the default
> value is still nil.

There is a change in what we are telling users is the proper form for the option
value.  Before, we made a big deal about them needing to use `^'; now we are
making a bid deal about them needing to use `\\`'.

At the very least:

1. We're telling users that _every_ value should be different from what _every_
value was supposed to be before. That's a change for users that they should be
made aware of, even if such a "should" is in fact vacuous wrt the difference.

2. We should tell users, in NEWS, that they do _not_ in fact need to change
existing values that respected the old rule - in spite of the changed rule.
They do _not_ need to swap \\` for ^.  That is not necessarily obvious to users.
 
> Can we close this bug, please?

Sure, after it's fixed.