emacs: Allow the user to choose the "From" address when replying to a message

When pressing C-u r, the user will be prompted for the identity to use.

emacs/notmuch-mua.el

@@ -69,7 +69,7 @@ list."
 	    (push header message-hidden-headers)))
 	notmuch-mua-hidden-headers))
 
-(defun notmuch-mua-reply (query-string)
+(defun notmuch-mua-reply (query-string &optional sender)
   (let (headers body)
     ;; This make assumptions about the output of `notmuch reply', but
     ;; really only that the headers come first followed by a blank
@@ -85,6 +85,9 @@ list."
 	      (setq headers (mail-header-extract)))))
       (forward-line 1)
       (setq body (buffer-substring (point) (point-max))))
+    ;; If sender is non-nil, set the From: header to its value.
+    (when sender
+      (mail-header-set 'from sender headers))
     (let
 	;; Overlay the composition window on that being used to read
 	;; the original message.
@@ -186,6 +189,14 @@ the From: address first."
 	(notmuch-mua-forward-message))
     (notmuch-mua-forward-message)))
 
+(defun notmuch-mua-new-reply (query-string &optional prompt-for-sender)
+  "Invoke the notmuch reply window."
+  (interactive "P")
+  (let ((sender
+	 (when prompt-for-sender
+	   (notmuch-mua-prompt-for-sender))))
+    (notmuch-mua-reply query-string sender)))
+
 (defun notmuch-mua-send-and-exit (&optional arg)
   (interactive "P")
   (message-send-and-exit arg))

emacs/notmuch-show.el

@@ -1098,10 +1098,10 @@ any effects from previous calls to
       ;; Move to the previous message.
       (notmuch-show-previous-message)))))
 
-(defun notmuch-show-reply ()
+(defun notmuch-show-reply (&optional prompt-for-sender)
   "Reply to the current message."
-  (interactive)
-  (notmuch-mua-reply (notmuch-show-get-message-id)))
+  (interactive "P")
+  (notmuch-mua-new-reply (notmuch-show-get-message-id) prompt-for-sender))
 
 (defun notmuch-show-forward-message (&optional prompt-for-sender)
   "Forward the current message."

emacs/notmuch.el

@@ -436,11 +436,11 @@ Complete list of currently available key bindings:
 			 32 nil nil t)))
       (error "End of search results"))))
 
-(defun notmuch-search-reply-to-thread ()
+(defun notmuch-search-reply-to-thread (&optional prompt-for-sender)
   "Begin composing a reply to the entire current thread in a new buffer."
-  (interactive)
+  (interactive "P")
   (let ((message-id (notmuch-search-find-thread-id)))
-    (notmuch-mua-reply message-id)))
+    (notmuch-mua-new-reply message-id prompt-for-sender)))
 
 (defun notmuch-call-notmuch-process (&rest args)
   "Synchronously invoke \"notmuch\" with the given list of arguments.