[Patch V4] Add NOTMUCH_MESSAGE_FLAG_EXCLUDED flag

[Mark Walters]

Here is the latest version of this patch set. I think I have fixed most
of the problems raised in review but there are some remaining issues
detailed below.

Changes and queries:

1) Changed --do-not-exclude option to --no-exclude

2) The api notmuch_query_set_omit_excluded_messages remains: without it I
can't see how a user can pass the notmuch_messages_t object around which
does not contain the excluded messages. See
id:"87fweusabh.fsf at qmul.ac.uk"

3) I have introduced a new function notmuch_thread_get_flag_messages
(notmuch_thread_t *thread, unsigned int flag_mask, unsigned int flags)

which returns the number of messages with the specified flags on
flag_mask. (Note the current NOTMUCH_MESSAGE_FLAGs were nominally the
bit position of the flag rather than the actual bit of the flag. I
changed that. I am not completely happy with the style for this commit
(patch 7/11): any comments gratefully received!

4) In id:"20120131044352.GZ17991 at mit.edu" Austin suggested that I use a
notmuch_mset_messages_t object rather than an notmuch_doc_id_set_t. I
couldn't see how that would work unless the iterator would generate the
excludes in step with the main query. At the moment the doc_id object
just stores a bitmap listing all relevant excluded messages.

5) If we have a query which overrides the excludes such as "blah and
tag:deleted" should the tag:deleted messages still be marked excluded?
The current implementation does mark them excluded but my preference would
be not to. What do people think?

6) In id:"20120131050748.GA10844 at mit.edu" Austin pointed out that the
sort will be influenced by the excluded messages. I do not think either
of us are sure whether it should be or not so I have left it as is for
the moment.

Best wishes

Mark