mu4e: limit # of references to 21

Following the example of `message-shorten-references', only allow up to
21 references, since some mail servers limit them.

Fixes #859.

mu4e/mu4e-draft.el

@@ -111,6 +111,16 @@ If VAL is nil, return nil."
 	      (propertize hdr 'face 'mu4e-header-key-face)
 	      (propertize val 'face 'mu4e-header-val-face))))
 
+(defconst mu4e~max-reference-num 21
+  "Maximum number of References:, as suggested by
+`message-shorten-references'.")
+
+(defun mu4e~shorten-1 (list cut surplus)
+  "Cut SURPLUS elements out of LIST, beginning with CUTth
+one. Code borrowed from `message-shorten-1'."
+  (setcdr (nthcdr (- cut 2) list)
+	  (nthcdr (+ (- cut 2) surplus 1) list)))
+
 (defun mu4e~draft-references-construct (msg)
   "Construct the value of the References: header based on MSG as a
 comma-separated string. Normally, this the concatenation of the
@@ -125,8 +135,14 @@ References. If both are empty, return nil."
 	  (refs (if (and msgid (not (string= msgid "")))
 		  (append refs (list msgid)) refs))
 	  ;; no doubles
-	  (refs (delete-duplicates refs :test #'equal)))
+	  (refs (delete-duplicates refs :test #'equal))
-    (mapconcat (lambda (id) (format "<%s>" id)) refs " ")))
+	  (refnum (length refs))
+	  (cut 2))
+    ;; remove some refs when there are too many
+    (when (> refnum mu4e~max-reference-num)
+      (let ((surplus (- refnum mu4e~max-reference-num)))
+	(mu4e~shorten-1 refs cut surplus))
+    (mapconcat (lambda (id) (format "<%s>" id)) refs " "))))
 
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -350,7 +366,8 @@ fields will be the same as in the original."
 
 	;; if there's no-one in To, copy the CC-list
 	(if (zerop (length (mu4e~draft-create-to-lst origmsg)))
-	  (mu4e~draft-header "To" (mu4e~draft-recipients-construct :cc origmsg reply-all))
+	  (mu4e~draft-header "To" (mu4e~draft-recipients-construct
+				    :cc origmsg reply-all))
 	  ;; otherwise...
 	  (concat
 	    (mu4e~draft-header "To" (mu4e~draft-recipients-construct :to origmsg))