send-email: add --[no-]xmailer option

Add --[no-]xmailer that allows a user to disable adding the 'X-Mailer:'
header to the email being sent.

Signed-off-by: Luis Henriques <henrix@camandro.org>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
test/send-email: --[no-]xmailer tests

Add tests for the --[no-]xmailer option.

Signed-off-by: Luis Henriques <henrix@camandro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>