Start on demo script making repo with NTFS stream

The repo this script makes attempts to check out entries traversing
the default `$INDEX_ALLOCATION` directory stream of the `.git`
directory, whose stream name is documented to be `$I30`.

However, although I am able to access directories under this naming
scheme through other applications, the repositories this script
currently creates do not appear to trigger the bug in gitoxide. The
next step is to try specifying the stream type explicitly.

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_ntfs_stream.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+set -eu
+
+repo="$1"
+git init -- "$repo"
+cd -- "$repo"
+
+# shellcheck disable=SC2016
+target_dir='subdir/.git:$I30/hooks'
+target_dir_standin="$(printf '%s' "$target_dir" | sed 's|:|,|g')"
+target_file="$target_dir/pre-commit"
+target_file_standin="$target_dir_standin/pre-commit"
+
+mkdir -p -- "$target_dir_standin"
+
+cat >"$target_file_standin" <<'EOF'
+#!/bin/sh
+printf 'Vulnerable!\n'
+date >vulnerable
+EOF
+
+git add --chmod=+x -- "$target_file_standin"
+
+standin_pattern="$(printf '%s' "$target_file_standin" | sed 's|[.$]|\\&|g')"
+cp .git/index old_index
+LC_ALL=C sed "s|$standin_pattern|$target_file|g" old_index >.git/index
+
+git commit -m 'Initial commit'