emacs: Add auto-tagging for replied messages.

Add `notmuch-message-mark-replied', a function for automatically tagging
replied messages with user-defined tags. The tags (which can be either
added or removed) can be customized with the customization variable
`notmuch-message-replied-tags'. This is a simple list of strings. Any
string prefaced with a "-" will be removed; any string prefaced with a "+"
(or neither "+" nor "-") will be added.

This adds a new file notmuch-message.el, for functions which target
message mode (and in the future, notmuch-message mode). Based on some
conversation, notmuch-message.el will probably end up subsuming
notmuch-mua.el, but until we figure out exactly how we want to do that,
they will remain separate files.

Edited-by: Carl Worth <cworth@cworth.org>: Remove trailing whitespace
and add newline at end of file.

emacs/notmuch-message.el

@@ -0,0 +1,52 @@
+;; notmuch-message.el --- message-mode functions specific to notmuch
+;;
+;; Copyright © Jesse Rosenthal
+;;
+;; This file is part of Notmuch.
+;;
+;; Notmuch is free software: you can redistribute it and/or modify it
+;; under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; Notmuch is distributed in the hope that it will be useful, but
+;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;; General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with Notmuch.  If not, see <http://www.gnu.org/licenses/>.
+;;
+;; Authors: Jesse Rosenthal <jrosenthal@jhu.edu>
+
+(require 'message)
+(require 'notmuch-mua)
+
+(defcustom notmuch-message-replied-tags '("replied")
+  "Tags to be automatically added to or removed from a message when it is replied to.
+Any tag in the list will be added to a replied message or,
+if it is prefaced with a \"-\", removed.
+
+For example, if you wanted to add a \"replied\" tag and remove
+the \"inbox\" and \"todo\", you would set
+    (\"replied\" \"-inbox\" \"-todo\"\)"
+  :type 'list
+  :group 'notmuch)
+
+(defun notmuch-message-mark-replied ()
+  ;; get the in-reply-to header and parse it for the message id.
+  (let ((rep (mail-header-parse-addresses (message-field-value "In-Reply-To"))))
+    (when (and notmuch-message-replied-tags rep)
+      ;; add a "+" to any tag that is doesn't already begin with a "+"
+      ;; or "-"
+      (let ((tags (mapcar '(lambda (str)
+			     (if (not (string-match "^[+-]" str))
+				 (concat "+" str)
+			       str))
+			  notmuch-message-replied-tags)))
+	(apply 'notmuch-call-notmuch-process "tag"
+	       (append tags (list (concat "id:" (car (car rep)))) nil))))))
+
+(add-hook 'message-send-hook 'notmuch-message-mark-replied)
+
+(provide 'notmuch-message)