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,
standard p5302-pack-index.sh runs improved as follows:

  libgit2:  3.17(11.42+0.61)  ->   2.69(10.49+0.32), 15.1% faster
  redis:    5.88(15.77+0.63)  ->   4.98(14.08+0.39), 15.3% faster
  git.git: 11.22(38.31+1.29)  ->   9.69(35.36+0.64), 13.6% faster
  cpython: 32.64(117.70+4.39) ->  28.70(109.90+1.98), 12.1% faster
  linux:  276.95(793.14+39.51) -> 234.75(722.33+18.01), 15.2% faster

The linux p5302 number is from a single repeat; the others are from the
default three repeats.

End-to-end local full-clone spot checks also improved:

  libgit2:  4.99s ->   4.50s, 9.8% faster
  redis:    8.71s ->   7.90s, 9.3% faster
  git.git: 24.89s ->  23.57s, 5.3% faster
  cpython: 58.77s ->  53.84s, 8.4% faster
  linux:  549.21s -> 515.52s, 6.1% faster

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>

Author: Arijit Banerjee

builtin/index-pack.c

@@ -1212,7 +1212,6 @@ static void *threaded_second_pass(void *data)
 			list_add(&child->list, &work_head);
 			base_cache_used += child->size;
 			prune_base_data(NULL);
-			free_base_data(child);
 		} else if (child) {
 			/*
 			 * This child does not have its own children. It may be