Fixes #3664.

Summary

• Recognize both SHA-1 (40 hex) and SHA-256 (64 hex) all-zero object IDs in the pre-push hook protocol.
• Keep deletion-only pushes as no-op pre-push runs instead of constructing a diff against a zero object ID.
• Add regression coverage for SHA-256 branch deletion stdin while preserving the existing SHA-1 deletion behavior.

Verification

• python -m pytest tests\commands\hook_impl_test.py -k "zero_oid or pre_push" -q passed: 9 passed.
• python -m pytest tests\commands\hook_impl_test.py -k "not legacy" -q passed: 41 passed, 5 deselected.
• python -m compileall -q pre_commit\commands\hook_impl.py tests\commands\hook_impl_test.py passed.
• git diff --check passed.
• SKIP=mypy pre-commit run --files pre_commit\commands\hook_impl.py tests\commands\hook_impl_test.py passed all non-mypy hooks.

Windows-local notes:

• python -m pytest tests\commands\hook_impl_test.py -q reported 45 passed and 1 failed because test_run_legacy_executes_legacy_script needs bash, which is not available in this Windows environment.
• The full mypy pre-commit hook reported existing Windows/platform-specific errors in unrelated modules (pre_commit\color.py, pre_commit\xargs.py, pre_commit\languages\docker.py) while checking these files.