git-send-email: Fix authenticating on some servers when using TLS.

git-send-email: Fix authenticating on some servers when using TLS.

Send HELO again after a successful STARTTLS command to refresh the list of
extensions. These may be different to what is returned over a clear
connection (for example the AUTH command may be accepted over a secure
connection, but not over a clear connection).

Furthermore, this behaviour is recommended by RFC 2487
(http://www.ietf.org/rfc/rfc2487.txt).

Signed-off-by: Robert Shearman <robertshearman@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>