If you change the subject during a reply, GMail expects a new thread ID

[ThomasHabets]

Describe the bug

If you change the subject during a reply, GMail expects a new thread ID. cmdg doesn't provide one.

To Reproduce

Steps to reproduce the behavior:

1. Receive email
2. Open a reply
3. In the reply editor, change the subject line.
4. Go to the gmail UI.
5. Find the email. There's no trace of the new subject line, even though "view original" shows it.

Expected behavior

Gmail web UI to show this as a separate thread. Which means it needs a new whatever the thread ID is called, if and only if the subject line changes. Sigh.

Desktop (please complete the following information):

• OS: n/a