merge script: --squash, --ff from unborn branch are errors

Port v1.6.1-rc1~319 (provide more errors for the "merge into empty
head" case, 2008-08-21) to the example merge script.

Noticed by comparison with builtin merge.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-merge.sh

@@ -245,6 +245,10 @@ then
 		exit 1
 	fi
 
+	test "$squash" != t ||
+		die "Squash commit into empty head not supported yet"
+	test "$allow_fast_forward" = t ||
+		die "Non-fast-forward into an empty head does not make sense"
 	rh=$(git rev-parse --verify "$1^0") ||
 		die "$1 - not something we can merge"