After the next patch, git-send-email will sometimes modify
existing Content-Transfer-Encoding headers. Delay the addition
of the header to @xh until just before sending. Do the same
for MIME-Version, to avoid adding it twice.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>