git-send-email: show all headers when sending mail

git-send-email: show all headers when sending mail

As a git newbie, it was confusing to set an In-Reply-To header but then
not see it printed when the git-send-email command was run.

This patch prints all headers that would be sent to sendmail or an SMTP
server instead of only printing From, Subject, Cc, To.  It also removes
the now-extraneous Date header after the "Log says" line.

Added test to t/t9001-send-email.sh.

Signed-off-by: David D. Kilzer <ddkilzer@kilzer.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>