Timing out a bounded git probe must not leave helper descendants
(credential helpers, git-remote-https, hook children) running after the
probe fails open. bounded_git_probe now spawns the child in its own
process group on POSIX (process_group=0), and _kill_git_process_tree
signals the whole group with os.killpg — gated on the child actually
leading its own group (pgid == pid), so a shared-group spawn can never
take down unrelated processes. Windows keeps the existing taskkill /T /F
tree kill.
Proven live on main: a fake git that forks a 300s descendant left the
descendant running after the probe timeout; with the fix the descendant
dies with the launcher. Fast path and fail-open contract unchanged.
Port of openai/codex#36793 (Terminate timed-out Git process trees).
Teknium