Give run_git's config nulls a witness no -c reaches (improvise-3j70)

GIT_CONFIG_GLOBAL=/dev/null and GIT_CONFIG_NOSYSTEM=1 in run_git were
witnessed by a config excludesFile, which vwjj's -c
core.excludesFile=/dev/null outranks. So dropping either line passed
every test from vwjj onwards.

The hostile child's config, read as both GIT_CONFIG_GLOBAL and
GIT_CONFIG_SYSTEM, now also sets core.sharedRepository = nonsense.
Measured on git 2.55: git init and git add die on it, git ls-files
does not read it (so the walk, which does not null the user's config,
is unaffected), and run_git passes no -c for it. init.defaultObjectFormat
and init.defaultRefFormat were tried first; they only warn.

Dropping either env line is now red. Every other rival in the combined
test is unchanged, and there is still one child per test.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

tests/shipped_documents_load.rs

@@ -223,9 +223,10 @@ fn a_hostile_git_environment_changes_nothing() {
     let excludes = home.path().join("ignore");
     std::fs::write(&excludes, "*.gz\n").expect("writes");
     let config = home.path().join("gitconfig");
-    // improvise-vav0: the same config also sends every trace2 target to stdout.
+    // vav0: trace2 targets go to stdout. 3j70: a bad sharedRepository kills `init`
+    // and `add` but not `ls-files`, and no `-c` in run_git overrides it.
     let text = format!(
-        "[core]\n\texcludesFile = {}\n[trace2]\n\tnormalTarget = /dev/stdout\n\teventTarget = /dev/stdout\n\tperfTarget = /dev/stdout\n",
+        "[core]\n\texcludesFile = {}\n[trace2]\n\tnormalTarget = /dev/stdout\n\teventTarget = /dev/stdout\n\tperfTarget = /dev/stdout\n[core]\n\tsharedRepository = nonsense\n",
         excludes.display()
     );
     std::fs::write(&config, text).expect("writes");