bundle: default to SHA1 when reading bundle headers

We hit a segfault when trying to open a bundle via `git bundle
list-heads` when running outside of a repository. This is caused by
c8aed5e8da (repository: stop setting SHA1 as the default object hash,
2024-05-07), which stopped setting the default object hash so that
`the_hash_algo` is a `NULL` pointer when running outside of any repo.

This is only a symptom of a deeper issue though. Bundles default to the
SHA1 object format unless they advertise an "@object-format=" header.
Consequently, it has been wrong in the first place to use the object
format used by the current repository when parsing bundles. The
consequence is that trying to open a bundle that uses a different object
hash than the current repository will fail:

    $ git bundle list-heads sha1.bundle
    error: unrecognized header: ee4b540943284700a32591ad09f7e15bdeb2a10c HEAD (45)

Fix the bug by defaulting to the SHA1 object hash. We already handle the
"@object-format=" header as expected, so we don't need to adapt this
part.

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bundle.c

@@ -89,7 +89,12 @@ int read_bundle_header_fd(int fd, struct bundle_header *header,
 		goto abort;
 	}
 
-	header->hash_algo = the_hash_algo;
+	/*
+	 * The default hash format for bundles is SHA1, unless told otherwise
+	 * by an "object-format=" capability, which is being handled in
+	 * `parse_capability()`.
+	 */
+	header->hash_algo = &hash_algos[GIT_HASH_SHA1];
 
 	/* The bundle header ends with an empty line */
 	while (!strbuf_getwholeline_fd(&buf, fd, '\n') &&