t3305: make hash agnostic

When computing the fanout length, let's use test_oid to look up the
hexadecimal size of the hash in question instead of hard-coding a value.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3305-notes-fanout.sh

@@ -7,7 +7,7 @@ test_description='Test that adding/removing many notes triggers automatic fanout
 path_has_fanout() {
 	path=$1 &&
 	fanout=$2 &&
-	after_last_slash=$((40 - $fanout * 2)) &&
+	after_last_slash=$(($(test_oid hexsz) - $fanout * 2)) &&
 	echo $path | grep -q "^\([0-9a-f]\{2\}/\)\{$fanout\}[0-9a-f]\{$after_last_slash\}$"
 }