verify-pack: fall back to SHA-1 outside a repo

In c8aed5e (repository: stop setting SHA1 as the default object hash,
2024-05-07), we have stopped setting the default hash algorithm for
`the_repository`. Consequently, code that relies on `the_hash_algo` will
now crash when it hasn't explicitly been initialized, which may be the
case when running outside of a Git repository.

As the verify-pack command ought to be able to infer what algorithm
is used in the input file (and if the input file does not have such
an information, that by itself is a problem), and the command allows
an option to explicitly tell what algorithm to use in case it cannot
be guessed from the input file, in theory we shouldn't have to use
the default algorithm anywhere in the operation of the command, but
we fail fairly early in the process when run outside a repository
without any default algorithm set.

Resurrect the setting of the default algorithm just like we used to
do before 2.46.0

Reported-by: Ilya K <me@0upti.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/verify-pack.c

@@ -1,6 +1,7 @@
 #include "builtin.h"
 #include "config.h"
 #include "gettext.h"
+#include "hash.h"
 #include "run-command.h"
 #include "parse-options.h"
 #include "strbuf.h"
@@ -77,6 +78,9 @@ int cmd_verify_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	if (!the_hash_algo)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, verify_pack_options,
 			     verify_pack_usage, 0);

t/t5300-pack-object.sh

@@ -322,6 +322,10 @@ test_expect_success 'verify-pack catches a corrupted sum of the index file itsel
 	fi
 '
 
+test_expect_success 'verify-pack outside a repository' '
+	nongit git verify-pack -v "$(pwd)/test-1-${packname_1}.idx"
+'
+
 test_expect_success 'build pack index for an existing pack' '
 	cat test-1-${packname_1}.pack >test-3.pack &&
 	git index-pack -o tmp.idx test-3.pack &&