send-email: support separate Reply-To address

In some projects contributions from groups are only accepted from a
common group email address. But every individual may want to receive
replies to her own personal address. That's what we have 'Reply-To'
headers for in SMTP. So introduce an optional '--reply-to' command
line option.

This patch re-uses the $reply_to variable. This could break
out-of-tree patches!

Signed-off-by: Christian Ludwig <chrissicool@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9001-send-email.sh

@@ -199,6 +199,7 @@ Message-Id: MESSAGE-ID-STRING
 X-Mailer: X-MAILER-STRING
 In-Reply-To: <unique-message-id@example.com>
 References: <unique-message-id@example.com>
+Reply-To: Reply <reply@example.com>
 
 Result: OK
 EOF
@@ -291,6 +292,7 @@ test_expect_success $PREREQ 'Show all headers' '
 		--dry-run \
 		--suppress-cc=sob \
 		--from="Example <from@example.com>" \
+		--reply-to="Reply <reply@example.com>" \
 		--to=to@example.com \
 		--cc=cc@example.com \
 		--bcc=bcc@example.com \