test-tool synthesize: precompute pack for 4 GiB + 1

The synthesize helper hashes roughly 8 GiB of data through SHA-1 to
produce a 4 GiB + 1 pack (4 GiB for the pack checksum, 4 GiB for
the blob OID). Since the blob content is all NUL bytes, every byte
in the resulting pack file is deterministic for a given blob size and
hash algorithm.

Add a fast path that writes the pack from precomputed constants:
a 25-byte prefix (pack header, object header, zlib header, first
block header), the zero-filled bulk with periodic 5-byte deflate
block headers, and a 513-byte suffix (tree, two commits, empty tree,
pack SHA-1 checksum). This eliminates all SHA-1 and adler32
computation, making the helper purely I/O-bound.

The precomputed constants are stored in a struct fast_pack array
keyed by hash algorithm format_id, so that adding SHA-256 support
later requires only adding another array entry with its suffix.

The constants were generated by running the generic path and
extracting the non-zero bytes from the resulting pack file.

Benchmarks generating a 4 GiB + 1 pack (3 runs each, SHA1DC on
x86_64):

  generic path:   88s / 81s / 140s
  fast path:      14s / 13s / 15s

On CI, where t5608 currently takes 200-850 seconds depending on the
job, the fast path cuts the pack-generation phase from minutes to
seconds, leaving only the clone operations themselves.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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