Has the same functionality as the '--cc' option and 'format.cc'
configuration variable but for the "To:" email header. Half of the code to
support this was already there.
With email the To: header usually more important than the Cc: header.
[jc: tests are by Stephen Boyd]
Signed-off-by: Steven Drake <sdrake@xnet.co.nz> Signed-off-by: Stephen Boyd <bebarino@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>