Require merge commits merge branches on top of other merge commits

Specifically, require that the left branch (first restult of git
show -s --format=format:%P) is a signed merge commit, instead of
allowing either. This is fine for now, but might need to be relaxed
in the future.

Also fixes an out-of-file-descriptors issue by holding too many
open FDs writing to /dev/null

contrib/verify-commits/verify-commits.sh

@@ -28,9 +28,10 @@ IS_SIGNED () {
 	local PARENTS
 	PARENTS=$(git show -s --format=format:%P $1)
 	for PARENT in $PARENTS; do
-		if IS_SIGNED $PARENT > /dev/null; then
+		if IS_SIGNED $PARENT; then
 			return 0;
 		fi
+		break
 	done
 	if ! "$HAVE_FAILED"; then
 		echo "No parent of $1 was signed with a trusted key!" > /dev/stderr