[codex] Ignore fsmonitor config in Git metadata reads (#22652)

## Summary
- keep Git metadata/status subprocesses independent of repository
`core.fsmonitor` configuration
- preserve existing working-tree state reporting while making the helper
behavior more predictable
- add regression coverage for `get_has_changes` when a repository
defines an fsmonitor command

## Validation
- `cargo fmt --all`
- `cargo test -p codex-core test_get_has_changes_`
- `cargo test -p codex-git-utils`

codex-rs/git-utils/src/info.rs

@@ -375,6 +375,7 @@ async fn run_git_command_with_timeout(args: &[&str], cwd: &Path) -> Option<std::
     let mut command = Command::new("git");
     command
         .env("GIT_OPTIONAL_LOCKS", "0")
+        .args(["-c", "core.fsmonitor=false"])
         .args(args)
         .current_dir(cwd)
         .kill_on_drop(true);