odb: use size_t for object_info.sizep and the size APIs

When `js/objects-larger-than-4gb-on-windows` widened the streaming,
index-pack and unpack-objects code paths, in the interest of keeping the
patches somewhat reasonably-sized, it left the public ODB API still
typed in `unsigned long`. In particular `struct object_info::sizep` and
the four wrappers built on top of it (`odb_read_object`,
`odb_read_object_peeled`, `odb_read_object_info`, `odb_pretend_object`)
still return the unpacked size through `unsigned long *`, so on Windows
`cat-file -s` and the `git add` / `git status` paths for a >4 GiB blob
silently cap at 4 GiB.

Widen the field and the four wrappers. The previous commits already
widened the `unpack_entry()` cascade and pack-objects' in-core size
accessors, so most of the cascade arrives here with no further work: the
temporary shims in `packed_object_info_with_index_pos()` and in
`unpack_entry()`'s delta-base recovery path go away, the two
`SET_SIZE(entry, cast_size_t_to_ulong(canonical_size))` calls in
`check_object()` and the matching one in `drop_reused_delta()` collapse
to plain `SET_SIZE`, and `oe_get_size_slow()`'s tail
`cast_size_t_to_ulong()` is gone too.

What remains narrow are the boundaries this series does not
intend to touch: the diff, blame, textconv and fast-import machinery.

Even so, this patch is unfortunately quite large.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -324,7 +324,7 @@ static int do_read_blob(const struct object_id *oid, struct oid_stat *oid_stat,
 			size_t *size_out, char **data_out)
 {
 	enum object_type type;
-	unsigned long sz;
+	size_t sz;
 	char *data;
 
 	*size_out = 0;