t5308: make test work with SHA-256

This test needs multiple object IDs that have the same first byte.
Update the pack test code to generate a suitable packed value for
SHA-256.  Update the test to use this value when using SHA-256.

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/lib-pack.sh

@@ -93,6 +93,14 @@ pack_obj () {
 			;;
 		esac
 		;;
+	# blob containing "\3\326"
+	471819e8c52bf11513f100b2810a8aa0622d5cd3d1c913758a071dd4b3bad8fe)
+		case "$2" in
+		'')
+			printf '\062\170\234\143\276\006\000\000\336\000\332'
+			return
+			;;
+		esac
 	esac
 
 	# If it's not a delta, we can convince pack-objects to generate a pack