Drop the filter-override listing from the .git guard's object read (#512)

[renmengye]

Fixes part of #512. Before this change, every guarded kernel git call ran four subprocesses: the .git guard's own object read (cat-file -e), a git config --get-regexp filter-override listing for that read, another listing for the real command, and the real command. The listing for the guard's read is useless. cat-file -e only resolves an object in the store and never converts working-tree files, so no filter driver can run in it.

The change: the guard's read calls _git_env(None), with no root, so no listing runs. It keeps the rest of the git environment: global and system config pinned to /dev/null, GIT_NO_REPLACE_OBJECTS, core.attributesFile=/dev/null, and the SAFE_GIT_FLAGS argv. The real command's listing is untouched, so its position and freshness are unchanged: it still runs after the config sanitize and immediately before the command. Nothing is cached, and nothing that is refused or neutralized changes.

Measured on the slowest test, test_author_sleep_wake_keeps_its_snapshot_when_the_terminal_record_fails:

• 1,277 → 959 subprocesses
• tests/test_attempt.py runs about 12% faster (≈134 s → 117 s)

Production saves one subprocess on every guarded git call.

Rejected alternatives:

• A pure-Python object lookup instead of cat-file. That would weaken the guard, because cat-file -e inflates the object and rejects a corrupt one.
• A cross-call cache of the listing, keyed on the config file. It could be made airtight, but it is more machinery than one dropped subprocess is worth.

The remaining cost in #512 is the guard's cat-file itself and the per-call listing. Both are left open.

Built by GLM-5.3 (self-reviewed, with a call-site audit), security-reviewed by Codex (clean), final read and simplification by Claude Opus 5.5. GLM's version added a pairs parameter to _git_env, which turned out to be unnecessary.

[github-actions]

Round 1 — reviewed head 22f1981c — reviewer summarizer:hermes/gpt-5.6-terra over coverage+credentials+deployment+general+lifecycle+prose.

terra
Advisory findings from outerloop — the code owner decides. Reply to disagree; the outerloop:no-review label opts this PR out.

Verdict: nothing blocking — 1 advisory note.

1 finding attached to the lines below.

Merged verdict: one non-blocking coverage suggestion. Rejected findings: none.