git-send-email: Do not make @-less message ID

git-send-email: Do not make @-less message ID

When the original $from address fails to yield a valid-looking
e-mail address, we created a bogus looking message ID, formatted
like this:

	Message-Id: <11823357623688-git-send-email->

This commit fixes it by moving call to make_message_id() to
where it matters, namely, before the $message_id is needed to be
placed in the generated e-mail header; this has an important
side effect of making it clear that $from is already available.

Also throw in Sys::Hostname::hostname() just for fun, although I
suspect that the code would never trigger due to the modified
call sequence that makes sure $from is always available.  This
is based on a suggestion by Michael Hendricks.

Signed-off-by: Junio C Hamano <gitster@pobox.com>