emacs: wash: make word-wrap bound message width

emacs: wash: make word-wrap bound message width

Previously if notmuch-wash-wrap-lines-length was set then all messages
would be wrapped at this value (or window-width if that is
smaller). This was done regardless of the message's depth in a thread --
for example, if the n.w.w.l.l is 80 and the messages depth is 20
(so indented 20 by default) the messages text only got 60 characters
of space.

This commit changes that so a message always gets the full n.w.w.l.l
of width regardless of its indentation (unless that goes over
window-width of course).