Merge branch 'sa/send-email-dedup-some-headers'

When fed input that already has In-Reply-To: and/or References: headers and told to add the same information, "git send-email" added these headers separately, instead of appending to an existing one, which is a violation of the RFC. This has been corrected. * sa/send-email-dedup-some-headers: send-email: avoid duplicate In-Reply-To/References