gix: test rejection of objectFormat on version-0 repositories

Add fixtures with sha1 and sha256 variants and an open() test asserting
that a version-0 repository setting extensions.objectFormat is rejected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>

Author: 10ne1

gix/tests/fixtures/generated-archives/make_config_repos.tar

@@ -165,6 +165,24 @@ mkdir not-a-repo-with-files;
   touch this that
 )
 
+# objectFormat is a v1-only extension; a version-0 repo that sets it is invalid (even for sha1).
+# Reach that state with git config independently of the fixture's default hash: first make a valid
+# v1+objectFormat repo, then flip the version to 0 as the last write. The flip must come last because
+# git config refuses to run once the repo is already invalid (v0 with a v1-only extension).
+git init objectformat-without-v1-sha256
+(cd objectformat-without-v1-sha256
+  git config core.repositoryFormatVersion 1
+  git config extensions.objectFormat sha256
+  git config core.repositoryFormatVersion 0
+)
+
+git init objectformat-without-v1-sha1
+(cd objectformat-without-v1-sha1
+  git config core.repositoryFormatVersion 1
+  git config extensions.objectFormat sha1
+  git config core.repositoryFormatVersion 0
+)
+
 git init ssl-verify-disabled
 (cd ssl-verify-disabled
   git config http.sslVerify false

gix/tests/fixtures/generated-archives/make_config_repos_sha256.tar

@@ -354,6 +354,27 @@ mod not_a_repository {
     }
 }
 
+mod object_format_extension {
+    use crate::util::named_subrepo_opts;
+
+    #[test]
+    fn rejects_object_format_on_v0_repo() -> crate::Result {
+        // objectFormat on a version-0 repo is rejected, even for sha1, matching git.
+        for name in ["objectformat-without-v1-sha256", "objectformat-without-v1-sha1"] {
+            let err = named_subrepo_opts("make_config_repos.sh", name, gix::open::Options::isolated())
+                .expect_err("a v0 repository setting extensions.objectFormat must be rejected");
+            assert!(
+                matches!(
+                    err,
+                    gix::open::Error::Config(gix::config::Error::ObjectFormatRequiresV1)
+                ),
+                "objectFormat on a v0 repository must be rejected, got {err:?} for {name}"
+            );
+        }
+        Ok(())
+    }
+}
+
 mod open_path_as_is {
 
     use crate::util::{named_subrepo_opts, repo_opts};