repository: Fix RestoreCustomHooks test depending on the host's umask

In order to calculate the voting hash for transactional updates in the RestoreCustomHooks() RPC we walk all custom hooks and hash their contents, paths and permission bits. As our tests for this behaviour expect hardcoded hashes this means that the hashed data must be the same on every system that runs these tests. But given that we depend on the permission bits it means that we now implicitly rely on the host's umask to be the same for every system, or otherwise the tests will fail.

Fix this by setting an explicit umask in the testcase.