notmuch.el: Switch to using "notmuch reply" rather than message-reply.

This way we get to take advantage of the configuration of the user's
email addresses in notmuch, (rather than expecting the user to
configure all of their email addresses in message mode as well).

notmuch.el

@@ -225,10 +225,15 @@ buffer."
 (defun notmuch-show-reply ()
   "Begin composing a reply to the current message in a new buffer."
   (interactive)
-  (view-file (notmuch-show-get-filename))
-  (let ((buf (current-buffer)))
-    (message-reply)
-    (kill-buffer buf)))
+  (let ((message-id (notmuch-show-get-message-id)))
+    (switch-to-buffer (generate-new-buffer "notmuch-draft"))
+    (call-process "notmuch" nil t nil "reply" message-id)
+    (goto-char (point-min))
+    (if (re-search-forward "^$" nil t)
+	(progn
+	  (insert "--text follows this line--")
+	  (forward-line)))
+    (message-mode)))
 
 (defun notmuch-show-pipe-message (command)
   "Pipe the contents of the current message to the given command.