Fix flaky test tests/test_git_credential_helper.py::test_a_planted_hook_does_not_run[core.hooksPath-fetch]

The merge lane saw `tests/test_git_credential_helper.py::test_a_planted_hook_does_not_run[core.hooksPath-fetch]` fail and then pass on a rerun of the same tree. - Test: `tests/test_git_credential_helper.py::test_a_planted_hook_does_not_run[core.hooksPath-fetch]` - Tree: `97f5c44e1a99d35c0c115ffec4f12ddbf7381add` - Lane task: 7584 - Flake record: 01a151b7-9064-7d66-b55c-a72d5ca01d91 Make it deterministic; do not delete it unless it tests nothing.

Fixed XPASS flake: the planted hook writes its marker via a builtin (no fork/PATH), and an incomplete git op now fails loudly instead of reading as the fix (xfail raises=AssertionError). Lane-condition XPASS reproduced and resolved; quick gate green; PR #6130 queued.