totalCancel is idempotent

In this context totalCancel can be called multiple times,
so mention that it is idempotent (no need to wrap with once).

getparty.go

@@ -293,7 +293,7 @@ func (cmd *Cmd) Run(args []string, version, commit string) (err error) {
 					atomic.AddUint32(&doneCount, 1)
 				case p.Skip:
 					cmd.dlogger.Print("Dropping total bar")
-					totalCancel(true)
+					totalCancel(true) // totalCancel is idempotent
 				}
 			}()
 			return p.download(client, req, timeout, sleep)