format-patch: thread as reply to cover letter even with in-reply-to

format-patch: thread as reply to cover letter even with in-reply-to

Currently, format-patch --thread --cover-letter --in-reply-to $parent
makes all mails, including the cover letter, a reply to $parent.
However, we would want the reader to consider the cover letter above
all the patches.

This changes the semantics so that only the cover letter is a reply to
$parent, while all the patches are formatted as replies to the cover
letter.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>