[RFC PATCH 2/4] Add NOTMUCH_MESSAGE_FLAG_EXCLUDED flag

[Mark Walters]

Ok I now have a patch set which might be complete enough to be worth
reviewing. It is essentially complete and appears to work.

Things that still need doing: 
       updating the test suite. The series changes notmuch-show to
       output the exclude flag so several tests need updating. Of
       course, the new functionality needs some tests too.
       
       emacs/notmuch.el I think it would be nice to hide (make
       invisible) threads with no matching non-excluded messages (with a
       toggle for visibility) but that is definitely beyond my elisp
       skills.

The first patch of the series is not really part of the series: it adds
a --do-not-exclude option to tell the command not to exclude. I think
this is useful anyway, but it also simplifies behaviour decisions with
the excludes. For example notmuch count will only count matching
non-excluded messages but notmuch count --do-not-exclude will count 
all matching messages excluded or not.

One outstanding issue is that raised in
id:"20120124025331.GZ16740 at mit.edu". I will need to think about
that. 

Best wishes

Mark