Messages with two or more tags that make then excluded from searches can
make them difficult to find in Notmuch Emacs interface. Let's say we
have config:

    [search]
    exclude_tags=spam;deleted;

*notmuch-hello* buffer has "All tags" section which lists how many
messages there are for every tag. However, if a message has two
different excluding tags (like "spam" and "deleted") then that message
is not shown anywhere in the "All tags" list. The message count does not
show it and the message does not show in the search when user clicks the
tag.

My opinion:

 1. "notmuch count" command should have --exclude option like "notmuch
    search" and Emacs interface should use --exclude=false in "All tags"
    section.
 
 2. When clicking a tag in "All tags" section the search operation
    should use --exclude=false because in these searches user (most
    likely) wants all messages by a tag regardless of other excluding
    tags.

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