pack-objects: widen `mem_usage` and `try_delta()`'s out-param to `size_t`

The pair must move together because `find_deltas()` passes `&mem_usage` to `try_delta()`: widening either alone breaks the type match. `mem_usage` accumulates per-object byte counts already computed in `size_t` (`SIZE()` and `sizeof_delta_index()` reach here through `free_unpacked()`, now `size_t`), and was the last 32-bit-on-Windows narrowing point in the delta-window memory accounting chain. With this commit, that chain uses `size_t` consistently except for `sizeof_delta_index()`'s still-narrow return, whose value is bounded by `create_delta_index()`'s entries cap. `window_memory_limit` (config-driven via `git_config_ulong()`) stays `unsigned long`: it is only compared against `mem_usage` and promotes. Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>