fix #1676, take 2

Ensure the pagination window is full by making sure that every history item
gets a replay line in CHATHISTORY output, even TAGMSG.

irc/history/queries.go

@@ -4,6 +4,7 @@
 package history
 
 import (
+	"strings"
 	"time"
 )
 
@@ -77,3 +78,16 @@ func MinMaxAsc(after, before, cutoff time.Time) (min, max time.Time, ascending b
 	}
 	return after, before, ascending
 }
+
+// maps regular msgids from JOIN, etc. to a msgid suitable for attaching
+// to a HistServ message describing the JOIN. See #491 for some history.
+func HistservMungeMsgid(msgid string) string {
+	return "_" + msgid
+}
+
+// strips munging from a msgid. future schemes may not support a well-defined
+// mapping of munged msgids to true msgids, but munged msgids should always contain
+// a _, with metadata in front and data (possibly the true msgid) after.
+func NormalizeMsgid(msgid string) string {
+	return strings.TrimPrefix(msgid, "_")
+}