[notmuch] notmuch.el: Prefix arg inverts the sort order of notmuch-search.

[racin at free.fr]

>On Thu, 11 Feb 2010 15:20:54 +0100 (CET), racin at free.fr wrote:
>
>> Using a prefix arg to invert search order would conflict with my patch
>> http://notmuchmail.org/pipermail/notmuch/2009/000914.html in which the
>> prefix arg is used to show deleted messages. I don't known which
>> behaviour for prefix patch would be best.
>
>I always understood a prefix arg to invert the original (or provide a
>numeric repetition), but I do not have a wide sample to tell if this is
>how most elisp implements it. 

Gnus uses a C-u prefix to display old messages, for instance. 

Note that the only advantage of the prefix over the toggle is that it makes one call to notmuch search instead of one. I don't think that it matters from a performance point of view, as the second search is always much faster than the second. Maybe we should just reserve the prefix for a later use.