test-tool synthesize: use the unsafe hash for speed

Jeff King pointed out on the mailing list [1] that t5608's new >4GB
test cases dominate the entire test suite runtime: 160 seconds on his
laptop when the rest of the suite finishes in under 90 seconds, and
305-850 seconds across CI jobs. The bottleneck is that the synthesize
helper hashes roughly 8 GB of data through SHA-1 (4 GB for the pack
checksum plus 4 GB for the blob OID) for a 4 GB+1 blob.

Since the helper generates known test data, collision detection is
unnecessary. Switch from repo->hash_algo to unsafe_hash_algo(), which
uses hardware-accelerated SHA-1 (via OpenSSL or Apple CommonCrypto)
when available.

Benchmarks on an x86_64 machine generating a 4 GB+1 pack (2 runs
each, interleaved):

  SHA-1 backend      Run 1    Run 2
  SHA1DC (safe)       75s      80s
  OpenSSL (unsafe)    21s      19s

The effect scales linearly. At 64 MB with 10 randomized interleaved
runs, the OpenSSL unsafe backend shows a 5.4x improvement (median
0.202s vs 1.088s) with tight variance (stdev 0.028s vs 0.095s).

The speedup is only realized when the build has a fast unsafe backend
compiled in. The CI's linux-TEST-vars job already sets
OPENSSL_SHA1_UNSAFE=YesPlease; macOS benefits from Apple CommonCrypto
when configured. On builds without a separate unsafe backend (such as
the default Windows builds), unsafe_hash_algo() returns the regular
collision-detecting implementation and the change is a no-op.

[1] https://lore.kernel.org/git/20260501063805.GA2038915@coredump.intra.peff.net/

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

t/helper/test-synthesize.c

@@ -217,7 +217,7 @@ static int cmd__synthesize__pack(int argc, const char **argv,
 
 	setup_git_directory_gently(&non_git);
 	repo = the_repository;
-	algo = repo->hash_algo;
+	algo = unsafe_hash_algo(repo->hash_algo);
 
 	argc = parse_options(argc, argv, NULL, options, usage,
 			     PARSE_OPT_KEEP_ARGV0);