t5703: feed raw data into test-tool unpack-sideband

busybox's sed isn't binary clean.
Thus, triggers false-negative on this test.

We could replace sed with perl on this usecase.
But, we could slightly modify the helper to discard unwanted data in the
beginning.

Fix the false negative by updating this helper.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-pkt-line.c

@@ -67,7 +67,7 @@ static void unpack_sideband(void)
 		case PACKET_READ_NORMAL:
 			band = reader.line[0] & 0xff;
 			if (band < 1 || band > 2)
-				die("unexpected side band %d", band);
+				continue; /* skip non-sideband packets */
 			fd = band;
 
 			write_or_die(fd, reader.line + 1, reader.pktlen - 1);

t/t5703-upload-pack-ref-in-want.sh

@@ -13,10 +13,7 @@ get_actual_refs () {
 }
 
 get_actual_commits () {
-	sed -n -e '/packfile/,/0000/{
-		/packfile/d
-		p
-		}' <out | test-tool pkt-line unpack-sideband >o.pack &&
+	test-tool pkt-line unpack-sideband <out >o.pack &&
 	git index-pack o.pack &&
 	git verify-pack -v o.idx >objs &&
 	grep commit objs | cut -d" " -f1 | sort >actual_commits