Add module-level _BASE_ENV_LOCK: threading.Lock and replace the bare
if _BASE_ENV is None assignment with double-checked locking. The outer
check keeps the warm-cache path lock-free; the inner check inside
with _BASE_ENV_LOCK prevents duplicate initialisation when two threads
race on the very first call.
Add three BDD scenarios in features/git_tools.feature (step definitions
in features/steps/git_tools_thread_safety_steps.py) verifying caching
identity, content correctness, and thread safety under 20 concurrent
threads using threading.Barrier.
Update CHANGELOG.md and CONTRIBUTORS.md per contribution guidelines.
ISSUES CLOSED: #7619
HAL9000