notmuch.el: Avoid infinite loop marking up message with no parts.

notmuch.el: Avoid infinite loop marking up message with no parts.

The infinite loop was triggered by a message consisting of a single
attachment within the body, (and no "part") tags.

We need to do things in response to this bug (beyond this specific
fix):

1. Create a test suite that exercises our emacs frontend so that bugs
   like this do not come back to haunt us after we fix them once.

2. Switch from our ad-hoc regexp based search of message-part delimeters
   to known-good code for parsing a structured document, (for example,
   the outstanding JSON patches).