which is the commit message and the patch itself in the
second part, with "Content-Disposition: inline".
---thread::
+--thread[=<style>]::
Add In-Reply-To and References headers to make the second and
subsequent mails appear as replies to the first. Also generates
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
+series, where the head is chosen from the cover letter, the
+`\--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.
--in-reply-to=Message-Id::
Make the first mail (or all the mails with --no-thread) appear as a