test: pin git background gc off in isolated test env (COR-394)

GitIsolatedEnv/IsolateGitConfigEnv now point git at a config that disables gc.auto, gc.autoDetach, maintenance.auto, and fetch.writeCommitGraph. A detached git gc/git maintenance process could outlive a test while still holding .git/objects open, racing t.TempDir()'s deferred RemoveAll and failing with "directory not empty".

Fixes flaky TestSafelyAdvanceLocalRef_DoesNotReplayDisconnectedChainWhenTargetIsShallow; applied centrally so other git-shelling strategy tests don't regress.