.dir-locals.el: spell out nil settings

.dir-locals.el: spell out nil settings

.dir-locals.el establishes our formatting conventions when writing and
formatting stuff with Emacs.  Since it has been written from within
Emacs using M-x add-dir-local-variable RET it spelled
(indent-tabs-mode . nil) as (indent-tabs-mode) which is confusing to
human readers.  So the file has been changed in the repo to the more
explicit form.  While this will not survive further applications of
M-x add-dir-local-variable RET, users are then responsible for their
own differences.

[lemzwerg, 2020-02-25 11:29:35]

LGTM.  Please directly push.

[dak, 2020-02-25 12:57:38]

On 2020/02/25 11:29:35, lemzwerg wrote:
> LGTM.  Please directly push.

I think the reason I could answer this more or less off-the-cuff was that it
confused the heck out of me until I figured out the reason.  So my fear is that
when people now use M-x add-directory-local-variable RET for adding more
settings, they will think it broken for removing the . nil stanza.  So I am
pretty sure I decided to pay that cost upfront in the default rather than having
users pay it with interest once they dare using some of the better Emacs
features.

It was sort of an on-the-fence decision for me, enough to have your request tip
it the other direction.  But I think it is fair, when touching this, to give
others a chance to weigh in, too.

It's not urgent, is it?

[lemzwerg, 2020-02-25 13:20:30]

> It's not urgent, is it?

No, not at all.  It just appeared to be a trivial change.