[18.0][FIX] odoo_repository: GitPython 3.1.59 compatibility

[lmignon]

GitPython 3.1.59, released on 2026-08-10, added '--separate-git-dir' to the list of unsafe options.

Allow unsafe options on clone created with workaround_fs_errors=True since they are the only once using this option.

Without this fix tests for `workaround_fs_error=Truer fails

[7](https://github.com/OCA/module-composition-analysis/actions/runs/31675398000/job/94368767381?pr=157#step:8:328)
2026-08-13 06:52:41,227 312 ERROR odoo odoo.addons.odoo_repository.tests.test_repository_scanner: ERROR: TestRepositoryScanner.test_workaround_fs_errors
Traceback (most recent call last):
  File "/__w/module-composition-analysis/module-composition-analysis/odoo_repository/tests/test_repository_scanner.py", line 215, in test_workaround_fs_errors
    scanner.sync()
  File "/__w/module-composition-analysis/module-composition-analysis/odoo_repository/lib/scanner.py", line 108, in sync
    res = self._clone()
  File "/__w/module-composition-analysis/module-composition-analysis/odoo_repository/lib/scanner.py", line 252, in _clone
    git.Repo.clone_from(**params)
  File "/opt/odoo-venv/lib/python3.10/site-packages/git/repo/base.py", line 1676, in clone_from
    return cls._clone(
  File "/opt/odoo-venv/lib/python3.10/site-packages/git/repo/base.py", line 1515, in _clone
    Git.check_unsafe_options(
  File "/opt/odoo-venv/lib/python3.10/site-packages/git/cmd.py", line 1025, in check_unsafe_options
    raise UnsafeOptionError(f"{unsafe_option} is not allowed, use `allow_unsafe_options=True` to allow it.")
git.exc.UnsafeOptionError: --separate-git-dir is not allowed, use `allow_unsafe_options=True` to allow it.