Make test_directory_bogus_perms/test_revision_bogus_perms actually test the cookers
Bogus permissions were normalized by git because it was even loaded; so the cooker's normalization was not actually tested.
This commit uses Dulwich instead of git to stage the changes, and monkey-patches Dulwich to prevent it from normalizing.
Migrated from D5689 (view on Phabricator)