You will also need to go and find the Message-Id of your previous cover letter.
You can either note it when you send the first series, from the output of `git
send-email`, or you can look it up on the
-https://public-inbox.org/git[mailing list]. Find your cover letter in the
+https://lore.kernel.org/git[mailing list]. Find your cover letter in the
archives, click on it, then click "permalink" or "raw" to reveal the Message-Id
header. It should match: