t-reftable-block: use reftable_record_key() instead of strbuf_addstr()

In the current testing setup, the record key required for many block
iterator functions is manually stored in a strbuf struct and then
passed to these functions. This is not ideal when there exists a
dedicated function to encode a record's key into a strbuf, namely
reftable_record_key(). Use this function instead of manual encoding.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Chandra Pratap

t/unit-tests/t-reftable-block.c

@@ -83,8 +83,7 @@ static void t_block_read_write(void)
 
 	for (i = 0; i < N; i++) {
 		struct block_iter it = BLOCK_ITER_INIT;
-		strbuf_reset(&want);
-		strbuf_addstr(&want, recs[i].u.ref.refname);
+		reftable_record_key(&recs[i], &want);
 
 		ret = block_iter_seek_key(&it, &br, &want);
 		check_int(ret, ==, 0);