[Bug] Notmuch-emacs show-whitespace interaction

[frozencemetery]

Hello,

When using (show-ws-toggle-show-trailing-whitespace) in the emacs
frontend on both emacs 23 and 24 with notmuch 0.15, the following
occurs:

> https://www.club.cc.cmu.edu/~rharwood/tmp/notmuch.png

To reproduce:

1. Open new buffer
2. Create new message with non-matching "-marks (this message will do)
3. M-x (show-ws-toggle-show-trailing-whitespace)
4. refresh buffer (=)

and you should see results similar to the above with respect to
highlighting of the "-delimited string.
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