Fix assertions in `test_git_hook`

Fix `called_once()` assertions in `test_git_hook` to use the correct
`assert_called_once()` method.  The former does not exist, so it
evaluates to a mocked method in Python < 3.12, making the assert
meaningless, and it triggers an error in Python 3.12+.

While at it, split the mock into two because otherwise the test would
fail because two `hooks.git_hook()` calls imply two mock calls.