reftable/record: store "val1" hashes as static arrays

When reading ref records of type "val1", we store its object ID in an
allocated array. This results in an additional allocation for every
single ref record we read, which is rather inefficient especially when
iterating over refs.

Refactor the code to instead use an embedded array of `GIT_MAX_RAWSZ`
bytes. While this means that `struct ref_record` is bigger now, we
typically do not store all refs in an array anyway and instead only
handle a limited number of records at the same point in time.

Using `git show-ref --quiet` in a repository with ~350k refs this leads
to a significant drop in allocations. Before:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 2,116,683 allocs, 2,116,491 frees, 76,098,060 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 1,419,031 allocs, 1,418,839 frees, 62,145,036 bytes allocated

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

reftable/block_test.c

@@ -49,13 +49,11 @@ static void test_block_read_write(void)
 
 	for (i = 0; i < N; i++) {
 		char name[100];
-		uint8_t hash[GIT_SHA1_RAWSZ];
 		snprintf(name, sizeof(name), "branch%02d", i);
-		memset(hash, i, sizeof(hash));
 
 		rec.u.ref.refname = name;
 		rec.u.ref.value_type = REFTABLE_REF_VAL1;
-		rec.u.ref.value.val1 = hash;
+		memset(rec.u.ref.value.val1, i, GIT_SHA1_RAWSZ);
 
 		names[i] = xstrdup(name);
 		n = block_writer_add(&bw, &rec);