In handle_body only read a line if we don't already have one.

This prepares for detecting non-email patches that don't have
mail headers.  In which case we have already read the first
line so handle_body should not ignore it.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>