send-email: restore --in-reply-to superseding behavior

send-email: restore --in-reply-to superseding behavior

git send-email --in-reply-to= fails to override In-Reply-To email headers,
if they're present in the output of format-patch, even when explicitly
told to do so by the option --no-thread, which breaks the contract of the
command line switch option, per its man page.

"
   --in-reply-to=<identifier>
       Make the first mail (or all the mails with --no-thread) appear as
       a reply to the given Message-Id, which avoids breaking threads to
       provide a new patch series.
"

This patch fixes the aformentioned issue, by bringing --in-reply-to's old
overriding behavior back.

The test was donated by Carlo Marcelo Arenas Belón.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>