Null the default excludes and attributes files for scratch repos

improvise-vwjj, found reviewing #248. GIT_CONFIG_GLOBAL=/dev/null empties
the config files. It does not stop git falling back to
$XDG_CONFIG_HOME/git/ignore (or ~/.config/git/ignore) when
core.excludesFile is unset, and git/attributes has the same kind of
default. So a *.gz line in the user's default ignore file still failed
a_gzipped_document_is_walked.

run_git now passes -c core.excludesFile=/dev/null and
-c core.attributesFile=/dev/null. A -c argument outranks every config
file and every default path.

The new test re-runs the gzip shape test under an XDG_CONFIG_HOME whose
git/ignore holds *.gz, and again with a git/attributes line that breaks
git add. Each half was red on its own.

Stacked on #248 (fix/53zz-scratch-repo-env).

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

context/test-evidence/tui.md

@@ -545,6 +545,43 @@ Rivals:
 - Drop `GIT_CONFIG_NOSYSTEM=1`: red, in the system run. The system scope is
   there so that this half of the fix has a witness at all.
 
+### a_scratch_repo_ignores_the_users_default_ignore_and_attributes_files
+
+improvise-vwjj, found reviewing PR #248 at `f5cfc7ed`. The test above sets
+`core.excludesFile` *inside* a config file, so emptying the config files looks
+like a complete fix. It is not. When `core.excludesFile` is unset, git falls
+back to `$XDG_CONFIG_HOME/git/ignore`, or `~/.config/git/ignore` when
+`XDG_CONFIG_HOME` is unset. No config file is involved, so
+`GIT_CONFIG_GLOBAL=/dev/null` leaves the fallback in force. It is also the
+commonest place to keep a global ignore, because it needs no config line.
+`core.attributesFile` has the same kind of default, `git/attributes`.
+
+The test points `XDG_CONFIG_HOME` at a temp directory and re-runs the gzip
+shape test in a child twice:
+
+- once with `git/ignore` holding `*.gz`;
+- once with `git/attributes` holding `*.gz working-tree-encoding=UTF-16`.
+
+The attributes line is chosen because it breaks `git add`: the one-byte file
+cannot be decoded as UTF-16. An attribute that only changes line endings would
+leave the add succeeding, and the case would witness nothing.
+
+Before the fix (at `f5cfc7ed`) the ignore run failed at `git add`. With only
+`-c core.excludesFile=/dev/null` added, the attributes run failed. So each half
+of the fix is witnessed separately.
+
+The fix is two `-c` arguments in `run_git`: `core.excludesFile=/dev/null` and
+`core.attributesFile=/dev/null`. A `-c` argument outranks every config file and
+every default path, so the `~/.config` fallback is closed by the same two lines.
+This test sets only `XDG_CONFIG_HOME`, so the `~/.config` path is covered by
+that argument, not exercised.
+
+Rivals, each run against the fixed file:
+
+- Drop `core.excludesFile=/dev/null`: red, `fails under a default git/ignore`.
+- Drop `core.attributesFile=/dev/null`: red, `fails under a default
+  git/attributes`.
+
 ---
 
 ## `tests/web_demo_model.rs`

tests/shipped_documents_load.rs

@@ -74,6 +74,8 @@ fn run_git(root: &Path, args: &[&str]) {
     let status = git(root)
         .env("GIT_CONFIG_GLOBAL", "/dev/null")
         .env("GIT_CONFIG_NOSYSTEM", "1")
+        .args(["-c", "core.excludesFile=/dev/null"])
+        .args(["-c", "core.attributesFile=/dev/null"])
         .args(args)
         .status()
         .expect("git runs");
@@ -216,3 +218,26 @@ fn a_scratch_repo_ignores_the_users_global_and_system_excludes() {
         );
     }
 }
+
+/// improvise-vwjj: git's DEFAULT ignore and attributes files need no config line.
+/// Evidence: `context/test-evidence/tui.md`.
+#[test]
+fn a_scratch_repo_ignores_the_users_default_ignore_and_attributes_files() {
+    let files = [
+        ("ignore", "*.gz\n"),
+        ("attributes", "*.gz working-tree-encoding=UTF-16\n"),
+    ];
+    for (name, text) in files {
+        let xdg = tempfile::tempdir().expect("a temp dir");
+        write(xdg.path(), &format!("git/{name}"), text);
+        let out = rerun_with_env(
+            "a_gzipped_document_is_walked",
+            &[("XDG_CONFIG_HOME", xdg.path())],
+        );
+        assert!(
+            out.status.success(),
+            "the gzip shape test fails under a default git/{name}: {}",
+            String::from_utf8_lossy(&out.stdout)
+        );
+    }
+}