a37acd5ee3
everyGitSubprocessGoesThroughTheHermeticFactory counts ProcessBuilder("git",
...) call sites, so it catches a new helper built the old way, but a
reviewer proved it does not catch gitProcessBuilder itself being gutted:
removing pb.environment().putAll(hermeticEnv()) from inside the factory
leaves every call site unchanged, the count stays 2, and the whole
unpoisoned suite stays green.
Add gitProcessBuilderCarriesTheFullHermeticEnvironment, which inspects what
the factory actually hands to ProcessBuilder#start(): every hermetic key
present with the isolating value, and XDG_CONFIG_HOME pointed inside the
class's own throwaway directory rather than left unset or pointing at the
operator's real one. This fails the moment the hermetic environment stops
being applied, on any machine, with no poison needed. Keep the call-site
count check too — the two catch different regressions.