send-email: add --header-cmd, --no-header-cmd options

send-email: add --header-cmd, --no-header-cmd options

Sometimes, adding a header different than CC or TO is desirable; for
example, when using Debbugs, it is best to use 'X-Debbugs-Cc' headers
to keep people in CC; this is an example use case enabled by the new
'--header-cmd' option.

The header unfolding logic is extracted to a subroutine so that it can
be reused; a test is added for coverage.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>