[RFC PATCH v2 1/3] cli: notmuch-show changes to support pick

[Mark Walters]

This patch adds the following features to notmuch-show.c

a --headers-only option which just outputs the headers of a message
(i.e. it omits the bodies). This is not strictly needed by
notmuch-pick.el but it gives a speed-up of at least a factor of a two;
moreover it reduces the memory usage of the emacs part hugely.

a --sort=oldest-first|newest-first option to allow notmuch-pick.el to
choose the sort order.

a --thread=none option to tell it to return just the matching messages
with not threading information at all. This is used by notmuch-pick.el
to display an unthreaded message view (e.g., for an inbox). It is also
useful for displaying a single message in the pick view as show is much
faster at showing  a single message than the whole thread.

The other options for --thread of --thread=entire|match replace the
--entire-thread option.

Presumably this should really be split into three separate patches, but
advice on this is gratefully received!

Thanks

Mark