CLI: scan in log time order when using index (#1160)

This changes the cat command to do a log-time order scan when it is able
to use the index (e.g seekable reader).

This was the intent of the existing code (there is no other reason for
it to try to use the index), but it was not supplying both necessary
parameters.

go/cli/mcap/cmd/cat.go

@@ -168,8 +168,12 @@ func (w *jsonOutputWriter) writeMessage(
 
 func getReadOpts(useIndex bool) []mcap.ReadOpt {
 	topics := strings.FieldsFunc(catTopics, func(c rune) bool { return c == ',' })
-	opts := []mcap.ReadOpt{mcap.UsingIndex(useIndex), mcap.WithTopics(topics)}
 
+	opts := []mcap.ReadOpt{mcap.WithTopics(topics)}
+
+	if useIndex {
+		opts = append(opts, mcap.UsingIndex(true), mcap.InOrder(mcap.LogTimeOrder))
+	}
 	catStart := catStartNano
 	if catStartSec > 0 {
 		catStart = catStartSec * 1e9