reftable/stack: fix broken refnames in write_n_ref_tables()

The `write_n_ref_tables()` helper function writes N references in
separate tables. We never reset the computed name of those references
though, leading us to end up with unexpected names.

Fix this by resetting the buffer.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

Diff for: reftable/stack_test.c

@@ -125,6 +125,7 @@ static void write_n_ref_tables(struct reftable_stack *st,
 			.value_type = REFTABLE_REF_VAL1,
 		};
 
+		strbuf_reset(&buf);
 		strbuf_addf(&buf, "refs/heads/branch-%04u", (unsigned) i);
 		ref.refname = buf.buf;
 		set_test_hash(ref.value.val1, i);