Add "!github help" and fix command selection bug

The last command would always be chosen due to pointer fail.

src/github.com/matrix-org/go-neb/clients/clients.go

@@ -215,11 +215,11 @@ func (c *Clients) onMessageEvent(client *gomatrix.Client, event *gomatrix.Event)
 // response is appropriate.
 func runCommandForService(cmds []types.Command, event *gomatrix.Event, arguments []string) interface{} {
 	var bestMatch *types.Command
-	for _, command := range cmds {
+	for i, command := range cmds {
 		matches := command.Matches(arguments)
 		betterMatch := bestMatch == nil || len(bestMatch.Path) < len(command.Path)
 		if matches && betterMatch {
-			bestMatch = &command
+			bestMatch = &cmds[i]
 		}
 	}
 

src/github.com/matrix-org/go-neb/services/github/github.go

@@ -163,6 +163,15 @@ func (s *Service) Commands(cli *gomatrix.Client) []types.Command {
 				return s.cmdGithubCreate(roomID, userID, args)
 			},
 		},
+		types.Command{
+			Path: []string{"github", "help"},
+			Command: func(roomID, userID string, args []string) (interface{}, error) {
+				return &gomatrix.TextMessage{
+					"m.notice",
+					fmt.Sprintf(`!github create owner/repo "title text" "description text"`),
+				}, nil
+			},
+		},
 	}
 }