hook: add sign-off using "interpret-trailers"

The sample hook to prepare the commit message before a commit allows users to opt-in to add the sign-off to the commit message. The sign-off is added at a place that isn't consistent with the "-s" option of "git commit". Further, it could go out of view in certain cases. Add the sign-off in a way similar to "-s" option of "git commit" using git's interpret-trailers command. It works well in all cases except when the user invokes "git commit" without any arguments. In that case manually add a new line after the first line to ensure it's consistent with the output of "-s" option. Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>