emacs: Split wide window vertically for tree mode message pane

[David Bremner]

Prof Jayanth R Varma <jrvarma at gmail.com> writes:

> This patch modifies the function notmuch-tree-show-message-in in 
> notmuch-tree.el to split the window vertically while creating a 
> message pane in tree-mode if the window is wider than 160 (so that 
> after splitting, the message pane can show the standard 76 
> character plain text email lines without wrapping).

First, I'm sorry to take so long to review this.  A few comments.

1) I'm reluctant to add new features to notmuch without corresponding
   tests. Currently all of our emacs tests use a standard terminal width
   (I think 80). Perhaps Tomi knows how hard it would be to override for
   a single test.

2) I noticed that the horizontal splitting leads to truncation of tag
   display. So I think not everyone will want this (it would be pretty
   frustrating to widen the emacs frame to see a full tag list, just to
   have it truncated). One option would be to control the splitting by a
   variable specifying the minimum width at which to switch to horizonal
   splitting.

3) The commit message / NEWS talks about splitting the window
   vertically, but that seems contrary to the emacs terminology (as
   evinced by the source code in the patch)