pack-objects: widen `free_unpacked()` return to `size_t`

pack-objects: widen `free_unpacked()` return to `size_t`

`free_unpacked()` sums two byte counts: `sizeof_delta_index()` and
`SIZE(n->entry)`. The latter has been `size_t` since the prior topic
"More work supporting objects larger than 4GB on Windows" widened
`SIZE()`/`oe_size()` to `size_t`, so accumulating it into an `unsigned
long` return was a silent Windows-only truncation on a packing run with
many large objects.

The sole caller, `find_deltas()`, still holds its own `mem_usage` in an
`unsigned long` for now, and therefore still truncates silently.

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