index-pack: retain child bases in delta cache

When resolving a delta whose result has children of its own, index-pack adds the result to work_head, accounts its data in base_cache_used, and calls prune_base_data(). It then immediately frees that same data. This bypasses the existing delta base cache policy and can force later descendants to reconstruct the queued base again. Let the existing delta_base_cache_limit pruning policy decide whether to keep or evict the data instead. This does not add a new cache or increase the cache limit. The object data is already accounted in base_cache_used before prune_base_data() runs, and the existing pruning and base cleanup paths still release it. On a quiet Ubuntu 24.04 VM with 16 vCPUs, 32 GiB RAM, and local SSD, direct index-pack timings on single-pack Linux fixtures improved as follows: linux blobless: 69.17s -> 57.98s (16.2% faster), RSS flat linux full: 280.72s -> 236.32s (15.8% faster), RSS +1.9% Five-repeat medians on public repositories also improved: git.git: 12.31s -> 10.70s (13.1% faster) libgit2: 3.35s -> 2.88s (14.0% faster) redis: 6.52s -> 5.64s (13.5% faster) cpython: 33.02s -> 31.44s (4.8% faster) The standard p5302 perf test on a smaller git.git fixture was neutral: 5302.9 index-pack default threads: 11.21(38.07+1.33) -> 11.16(37.90+1.31), -0.4% t/t5302-pack-index.sh passed, and GitGitGadget's linux-leaks CI also exercised that test under SANITIZE=leak. Signed-off-by: Arijit Banerjee <arijit@effectiveailabs.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>