mu/mu4e: implement mu4e-compose-complete-max

Allow limiting number of contacts for auto-completion to a specific number;
defaulting to 2000.

mu4e/mu4e-contacts.el

@@ -56,6 +56,17 @@ time-based restriction."
   :type 'string
   :group 'mu4e-compose)
 
+(defcustom mu4e-compose-complete-max 2000
+  "Consider only the top-n contacts.
+After considering the other
+constraints (`mu4e-compose-complete-addresses' and
+`mu4e-compose-complete-only-after'), pick only the highest-ranked n.
+
+This reduce start-up time and memory usage. Set to nil for no
+limits."
+  :type 'string
+  :group 'mu4e-compose)
+
 ;; names and mail-addresses can be mapped onto their canonical
 ;; counterpart.  use the customizeable function
 ;; mu4e-canonical-contact-function to do that.  below the identity
@@ -293,6 +304,7 @@ nothing."
     (mu4e--server-contacts
      mu4e-compose-complete-only-personal
      mu4e-compose-complete-only-after
+     mu4e-compose-complete-max
      mu4e--contacts-tstamp)))
 
 (provide 'mu4e-contacts)

mu4e/mu4e-server.el

@@ -441,17 +441,21 @@ The result is delivered to the function registered as
      :decrypt ,(and decrypt t)
      :docid   ,docid)))
 
-(defun mu4e--server-contacts (personal after tstamp)
-  "Ask for contacts with PERSONAL AFTER TSTAMP.
-S-expression (:contacts (<list>) :tstamp \"<tstamp>\") is expected in
-response. If PERSONAL is non-nil, only get personal contacts, if
-AFTER is non-nil, get only contacts seen AFTER (the time_t
-value)."
+(defun mu4e--server-contacts (personal after maxnum tstamp)
+  "Ask for contacts with PERSONAL AFTER MAXNUM TSTAMP.
+
+S-expression (:contacts (<list>) :tstamp \"<tstamp>\")
+is expected in response.
+
+If PERSONAL is non-nil, only get personal contacts, if AFTER is
+non-nil, get only contacts seen AFTER (the time_t value). If MAX is non-nil,
+get at most MAX contacts."
   (mu4e--server-call-mu
    `(contacts
      :personal ,(and personal t)
      :after    ,(or after nil)
-     :tstamp   ,(or tstamp nil))))
+     :tstamp   ,(or tstamp nil)
+     :maxnum   ,(or maxnum nil))))
 
 (defun mu4e--server-find (query threads sortfield sortdir maxnum skip-dups
 				include-related)