emacs: wash: word-wrap bugfix

emacs: wash: word-wrap bugfix

Previously notmuch-wash made the width of the text (approximately) the
window-width minus the depth in thread. This is correct for the
default indentation of 1 per message depth, but is incorrect for any
other setting of notmuch-show-indent-messages-width.

As notmuch-show-indent-messages-width is customisable, and notmuch-tree
sets it to zero to avoid indenting messages in the message pane, this
bug can show up in real use.

Two of the tests had to be updated: when
notmuch-show-indent-messages-width is 0, then the new (correct) word
wrapping happens later, when notmuch-show-indent-messages-width is 4,
then the new word wrapping happens sooner.