sample commit-msg hook: no silent exit on duplicate Signed-off-by lines

git-commit would silently exit if duplicate Signed-off-by
lines were found.  Users of git-commit would not know it,
unless they checked '$?'.  This patch makes git-commit
actually print out a message that nothing was commited
since duplicate Signed-off-lines were found.

Signed-off-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>