index-pack: retain child bases in delta cache

[arijit91]

Speed up the local pack indexing phase of clone/fetch for large
delta-compressed packs by keeping reconstructed delta bases available for
reuse when they are queued for later delta resolution.

When index-pack reconstructs a child base and queues it for resolving
descendant deltas, it currently frees that data immediately. This can force
the same base to be reconstructed again. Instead, keep it in the existing
delta base cache and let the existing delta_base_cache_limit policy decide
whether to retain or evict it.

This does not add a new cache or increase the cache limit. The object data is
already accounted in base_cache_used, and prune_base_data() is already
called at this point.

Correctness:

• t/t5302-pack-index.sh passed all 36 tests.

Benchmarks on a quiet Ubuntu 24.04 VM, 16 vCPU, 32 GiB RAM, local SSD:

pack	baseline	patched	wall-time change	RSS change
linux blobless	69.17s	57.98s	16.2% faster	-0.0%
linux full	280.72s	236.32s	15.8% faster	+1.9%

Five-repeat public-repo medians also improved: git.git 13.1%, libgit2
14.0%, redis 13.5%, cpython 4.8%.

Perf on the linux blobless pack showed the same direction under profiling:
76.64s baseline vs 61.09s patched, with similar RSS.

CC: Ævar Arnfjörð Bjarmason avarab@gmail.com, Junio C Hamano gitster@pobox.com, Derrick Stolee stolee@gmail.com

[gitgitgadget]

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[arijit91]

/submit

[gitgitgadget]

Error: User arijit91 is not yet permitted to use GitGitGadget

[dscho]

/allow

[gitgitgadget]

User arijit91 is now allowed to use GitGitGadget.

[arijit91]

/submit

[gitgitgadget]

Submitted as pull.2131.git.1780070763044.gitgitgadget@gmail.com

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-2131/arijit91/index-pack-retain-child-base-v1

To fetch this version to local tag pr-2131/arijit91/index-pack-retain-child-base-v1:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-2131/arijit91/index-pack-retain-child-base-v1