emacs: Split wide window vertically for tree mode message pane

[Prof Jayanth R Varma]

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).
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