Add scrollback support for direct messages (DMs). Fixes #385 (#388)

* Add scrollback support for direct messages (DMs). Fixes #385

* Fixed based on review

mm-go-irckit/service.go

@@ -3,6 +3,7 @@ package irckit
 import (
 	"errors"
 	"fmt"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -263,27 +264,34 @@ func scrollback(u *User, toUser *User, args []string, service string) {
 	}
 
 	if len(args) != 2 {
-		u.MsgUser(toUser, "need SCROLLBACK <channel> <lines>")
+		u.MsgUser(toUser, "need SCROLLBACK (#<channel>|<user>) <lines>")
 		u.MsgUser(toUser, "e.g. SCROLLBACK #bugs 10 (show last 10 lines from #bugs)")
 		return
 	}
 
 	limit, err := strconv.Atoi(args[1])
 	if err != nil {
-		u.MsgUser(toUser, "need SCROLLBACK <channel> <lines>")
+		u.MsgUser(toUser, "need SCROLLBACK (#<channel>|<user>) <lines>")
 		u.MsgUser(toUser, "e.g. SCROLLBACK #bugs 10 (show last 10 lines from #bugs)")
 		return
 	}
 
-	if !strings.Contains(args[0], "#") {
-		u.MsgUser(toUser, "need SCROLLBACK <channel> <lines>")
+	var channelName string
+	if strings.HasPrefix(args[0], "#") {
+		channelName = strings.ReplaceAll(args[0], "#", "")
+	} else if scrollbackUser, exists := u.Srv.HasUser(args[0]); exists && scrollbackUser.Ghost {
+		// We need to sort the two user IDs to construct the DM
+		// channel name.
+		userIDs := []string{u.User, scrollbackUser.User}
+		sort.Strings(userIDs)
+		channelName = userIDs[0] + "__" + userIDs[1]
+	} else {
+		u.MsgUser(toUser, "need SCROLLBACK (#<channel>|<user>) <lines>")
 		u.MsgUser(toUser, "e.g. SCROLLBACK #bugs 10 (show last 10 lines from #bugs)")
 		return
 	}
 
-	args[0] = strings.ReplaceAll(args[0], "#", "")
-
-	list := u.br.GetPosts(u.br.GetChannelID(args[0], u.br.GetMe().TeamID), limit)
+	list := u.br.GetPosts(u.br.GetChannelID(channelName, u.br.GetMe().TeamID), limit)
 	if list == nil || list.(*model.PostList) == nil || len(list.(*model.PostList).Order) == 0 {
 		u.MsgUser(toUser, "no results")
 		return