send-email: avoid duplicate In-Reply-To/References

send-email: avoid duplicate In-Reply-To/References

In case a patch already has In-Reply-To or References in the header
(e.g. when the patch has been created with format-patch --thread)
git-send-email should not add another pair of those headers.
This is also not allowed according to RFC 5322 Section 3.6:
https://tools.ietf.org/html/rfc5322#section-3.6

Avoid the second pair by reading the current headers into the
appropriate variables.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>