If requested to signoff a commit, don't add another Signed-off-by: line
to the commit message if the exact same line is already there.
This was noticed and requested by Josh Triplett through
http://bugs.debian.org/430851
Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>