reftable: make assignments portable to AIX xlc v12.01

Change the assignment syntax introduced in 66c0dabab5 (reftable: make
reftable_record a tagged union, 2022-01-20) to be portable to AIX xlc
v12.1:

    avar@gcc111:[/home/avar]xlc -qversion
    IBM XL C/C++ for AIX, V12.1 (5765-J02, 5725-C72)
    Version: 12.01.0000.0000

The error emitted before this was e.g.:

    "reftable/generic.c", line 133.26: 1506-196 (S) Initialization
    between types "char*" and "struct reftable_ref_record" is not
    allowed.

The syntax in the pre-image is supported by e.g. xlc 13.01 on a newer
AIX version:

    avar@gcc119:[/home/avar]xlc -qversion
    IBM XL C/C++ for AIX, V13.1.3 (5725-C72, 5765-J07)
    Version: 13.01.0003.0006

But as we've otherwise supported this compiler let's not break it
entirely if it's easy to work around it.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/generic.c

@@ -130,7 +130,9 @@ int reftable_iterator_next_ref(struct reftable_iterator *it,
 {
 	struct reftable_record rec = {
 		.type = BLOCK_TYPE_REF,
-		.u.ref = *ref,
+		.u = {
+			.ref = *ref
+		},
 	};
 	int err = iterator_next(it, &rec);
 	*ref = rec.u.ref;
@@ -142,7 +144,9 @@ int reftable_iterator_next_log(struct reftable_iterator *it,
 {
 	struct reftable_record rec = {
 		.type = BLOCK_TYPE_LOG,
-		.u.log = *log,
+		.u = {
+			.log = *log,
+		},
 	};
 	int err = iterator_next(it, &rec);
 	*log = rec.u.log;

reftable/record_test.c

@@ -339,7 +339,9 @@ static void test_reftable_obj_record_roundtrip(void)
 		};
 		struct reftable_record in = {
 			.type = BLOCK_TYPE_OBJ,
-			.u.obj = recs[i],
+			.u = {
+				.obj = recs[i],
+			},
 		};
 		struct strbuf key = STRBUF_INIT;
 		struct reftable_record out = { .type = BLOCK_TYPE_OBJ };

reftable/writer.c

@@ -257,7 +257,9 @@ int reftable_writer_add_ref(struct reftable_writer *w,
 {
 	struct reftable_record rec = {
 		.type = BLOCK_TYPE_REF,
-		.u.ref = *ref,
+		.u = {
+			.ref = *ref
+		},
 	};
 	int err = 0;
 
@@ -308,7 +310,9 @@ static int reftable_writer_add_log_verbatim(struct reftable_writer *w,
 {
 	struct reftable_record rec = {
 		.type = BLOCK_TYPE_LOG,
-		.u.log = *log,
+		.u = {
+			.log = *log,
+		},
 	};
 	if (w->block_writer &&
 	    block_writer_type(w->block_writer) == BLOCK_TYPE_REF) {
@@ -401,7 +405,9 @@ static int writer_finish_section(struct reftable_writer *w)
 		for (i = 0; i < idx_len; i++) {
 			struct reftable_record rec = {
 				.type = BLOCK_TYPE_INDEX,
-				.u.idx = idx[i],
+				.u = {
+					.idx = idx[i],
+				},
 			};
 			if (block_writer_add(w->block_writer, &rec) == 0) {
 				continue;