Don't hash headers we won't end up using.

Just saving a little work here.

database.cc

@@ -486,6 +486,13 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
 
     message = notmuch_message_open (filename);
 
+    notmuch_message_restrict_headers (message,
+				      "references",
+				      "in-reply-to",
+				      "message-id",
+				      "date",
+				      (char *) NULL);
+
     try {
 	doc = Xapian::Document ();
 

message.c

@@ -241,7 +241,7 @@ notmuch_message_get_header (notmuch_message_t *message,
     contains = g_hash_table_lookup_extended (message->headers,
 					     header_desired, NULL,
 					     (gpointer *) &value);
-    if (contains)
+    if (contains && value)
 	return value;
 
     if (message->parsing_finished)