Summary

Fix git_indexer_hash() returning an incorrectly represented object ID.

The existing implementation casts raw pack checksum bytes directly to git_oid *. With the current git_oid layout, the first checksum byte is incorrectly interpreted as the OID type.

This change:

• Stores the checksum in a properly initialized git_oid, using the indexer's object format.
• Returns the correctly typed OID from git_indexer_hash().
• Adds a focused regression assertion checking both the SHA-1 OID type and the expected checksum.

Regression evidence

Before the fix:

• pack::indexer::out_of_order fails.
• Expected OID type: GIT_OID_SHA1 (1).
• Actual OID type: 222 (0xde), the first checksum byte.
• Exit code: 1.

After the fix:

• The focused regression test passes.
• Clar reports ok 1 - pack::indexer::out_of_order.
• Exit code: 0.

Validation

• Built libgit2_tests using Visual Studio 2022 MSVC, x64, Debug configuration.
• Build succeeded with parallelism limited to two jobs.
• Focused regression test passed on the updated upstream main base.
• git diff --check passed.

The regression test uses the existing SHA-1 pack fixture. SHA-256 compatibility is handled by the format-aware conversion but is not directly covered by this native regression test.

This correction was identified while testing libgit2 integration for rust-lang/git2-rs#1259.