packfile: widen unpack_entry()'s size out-parameter to size_t

packfile: widen unpack_entry()'s size out-parameter to size_t

The topic `js/objects-larger-than-4gb-on-windows` widened the streaming,
index-pack and unpack-objects paths to `size_t` but deliberately stopped
at the in-memory `unpack_entry()` cascade, which still hands back the
unpacked size through `unsigned long *`.  On Windows that boundary
truncates above 4 GiB because that data type is only 32 bits wide on
that platform.

Widen the code path. Except `packed_object_info_with_index_pos()`: It
cannot yet pass `oi->sizep` directly because the field is still
`unsigned long *`; bridge it with a `size_t` temporary that narrows
back, and let a later commit drop the bridge once the field is wide
too. `gfi_unpack_entry()` keeps its narrow signature because fast-import
tracks sizes through `unsigned long` everywhere it crosses subsystem
boundaries, keeping its signature allows the scope of this commit to be
somewhat reasonable, still.

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