the Message-Id header to reference.
+
The optional <style> argument can be either `shallow` or `deep`.
-'Shallow' threading makes every mail a reply to the head of the
+'shallow' threading makes every mail a reply to the head of the
series, where the head is chosen from the cover letter, the
-`\--in-reply-to`, and the first patch mail, in this order. 'Deep'
+`\--in-reply-to`, and the first patch mail, in this order. 'deep'
threading makes every mail a reply to the previous one. If not
specified, defaults to the 'format.thread' configuration, or `shallow`
if that is not set.