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