REQ-GIT-084: The test suite cannot reach any repository but its own fixtures, in any ambient git environment

Running the suite must leave every repository other than its own fixtures byte-identical, regardless of the git environment it inherits. In particular an inherited GIT_DIR, GIT_WORK_TREE or other redirect variable — which .githooks/pre-push exports into the mix test it runs — must not decide which repository a command operates on, and a git invocation must not fall back to the project root for want of a working directory. GIT_CEILING_DIRECTORIES guards only repository discovery and cannot cover either case: a redirect variable bypasses discovery, and a repository at the current directory requires no ascent.