pack-objects: widen delta-cache accounting to `size_t`

These three are a single accounting tuple (the globals tracking cumulative cached-delta bytes, plus the helper that compares them against an incoming delta size) and are latently 32-bit on Windows where `unsigned long` != `size_t`: a pack with many large cached deltas could wrap silently. The widening is internally consistent on its own: the additions and subtractions against delta_cache_size already come from `size_t` sources (`DELTA_SIZE()` returns `size_t`), and `delta_cacheable()`'s sole caller in `try_delta()` still passes `unsigned long`, which promotes. Prerequisite for dropping `try_delta()`'s `cast_size_t_to_ulong()` shims, which becomes possible once 1create_delta()` and `diff_delta()` are widened in a later commit. Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>