Authenticate only once in git-send-email

Authenticate only once in git-send-email

When using git-send-email with SMTP authentication sending a patch series
would redundantly authenticate multiple times, once for each patch. In
the worst case, this would actually prevent the series from being sent
because the server would reply with a "5.5.0 Already Authenticated"
status code which would derail the process.

This commit teaches git-send-email to authenticate once and only once at
the beginning of the series.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>