t-reftable-block: use xstrfmt() instead of xstrdup()

Use xstrfmt() to assign a formatted string to a ref record's
refname instead of xstrdup(). This helps save the overhead of
a local 'char' buffer as well as makes the test more compact.

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

@@ -44,10 +44,7 @@ static void t_block_read_write(void)
 	check_int(ret, ==, REFTABLE_API_ERROR);
 
 	for (i = 0; i < N; i++) {
-		char name[100];
-		snprintf(name, sizeof(name), "branch%02"PRIuMAX, (uintmax_t)i);
-
-		rec.u.ref.refname = xstrdup(name);
+		rec.u.ref.refname = xstrfmt("branch%02"PRIuMAX, (uintmax_t)i);
 		rec.u.ref.value_type = REFTABLE_REF_VAL1;
 		memset(rec.u.ref.value.val1, i, GIT_SHA1_RAWSZ);