This is like notmuch-search-filter-by-tag, but creates a new search
rather than filtering the current search. We add this to
notmuch-common-keymap since this can be used by many contexts. We bind
to the key "t", which is the same key used by
notmuch-search-filter-by-tag in notmuch-search-mode-map. This is done
intentionally since the keybinding for notmuch-search-mode-map can be
seen as a specialization of creating a new search.
This change was motivated for use in "notmuch-hello". It is a more
convenient way to search a tag than expanding the list of all tags. I
also noticed many saved searches people use are simply tags.
 | q         | notmuch-bury-or-kill-this-buffer       | notmuch-bury-or-kill-this-buffer                      | notmuch-bury-or-kill-this-buffer        |
 | r         | notmuch-search-reply-to-thread-sender  | notmuch-show-reply-sender                             | notmuch-show-reply-sender               |
 | s         | notmuch-search                         | notmuch-search                                        | notmuch-search                          |
-| t         | notmuch-search-filter-by-tag           | toggle-truncate-lines                                 |                                         |
+| t         | notmuch-search-filter-by-tag           | toggle-truncate-lines                                 | notmuch-search-by-tag                   |
 | u         |                                        |                                                       |                                         |
 | v         |                                        |                                                       | notmuch-show-view-all-mime-parts        |
 | w         |                                        | notmuch-show-save-attachments                         | notmuch-show-save-attachments           |
 
     (define-key map "?" 'notmuch-help)
     (define-key map "q" 'notmuch-bury-or-kill-this-buffer)
     (define-key map "s" 'notmuch-search)
+    (define-key map "t" 'notmuch-search-by-tag)
     (define-key map "z" 'notmuch-tree)
     (define-key map "u" 'notmuch-unthreaded)
     (define-key map "m" 'notmuch-mua-new-mail)
 
    (list (notmuch-select-tag-with-completion "Filter by tag: " notmuch-search-query-string)))
   (notmuch-search (concat notmuch-search-query-string " and tag:" tag) notmuch-search-oldest-first))
 
+(defun notmuch-search-by-tag (tag)
+  "Display threads matching TAG in a notmuch-search buffer."
+  (interactive
+   (list (notmuch-select-tag-with-completion "Notmuch search tag: ")))
+  (notmuch-search (concat "tag:" tag)))
+
 ;;;###autoload
 (defun notmuch ()
   "Run notmuch and display saved searches, known tags, etc."