Combine non-"slashes" (i.e. trees) scripts and make it a fixture

At least for now, this does not test the creation of multiple files
at a time outside of a repository, nor multi-step upward traversal
with many `../../..` components, since tests using such fixtures
would be complicated, and may or may not be warranted in the test
suite.

However, this combines substantial elements of the scripts that
create repositories with unexpected tree objects (e.g., `..` trees)
to make a make_traverse_trees.sh script that, when run, produces
repositories for testing that traverse:

- Upward with a `..` tree: `traverse_dotdot_tree`
- Downward with `.git` and `hooks` trees: `traverse_dotgit_trees`
- Similar but with an NTFS stream alias: `traverse_dotgit_stream`

This replaces the `make_traverse_dotdot_trees.sh` and
`make_traverse_ntfs_streams.sh` scripts with one script that takes
no command-line arguments and creates multiple repos by calling
a function. This is thus architecturally similar, broadly speaking,
to `make_traverse_literal_slashes.sh`, but that produces repos with
very strangely named blobs, rather than with strangly named trees.

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_dotdot_trees.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+set -eu -o pipefail
+
+# Makes a repo carrying a tree structure representing the given path to a blob.
+# File content is from stdin. Args are repo name, path, -x or +x, and tr sets.
+function make_repo() (
+  local repo="$1" path="$2" xbit="$3" set1="$4" set2="$5"
+  local dir dir_standin path_standin path_standin_pattern path_replacement
+
+  git init -- "$repo"
+  cd -- "$repo" # Temporary, as the function body is a ( ) subshell.
+
+  dir="${path%/*}"
+  dir_standin="$(tr "$set1" "$set2" <<<"$dir")"
+  path_standin="$(tr "$set1" "$set2" <<<"$path")"
+  mkdir -p -- "$dir_standin"
+  cat >"$path_standin"
+  git add --chmod="$xbit" -- "$path_standin"
+  path_standin_pattern="$(sed 's/[|.*^$\]/\\&/g' <<<"$path_standin")"
+  path_replacement="$(sed 's/[|&\]/\\&/g' <<<"$path")"
+  cp .git/index old_index
+  LC_ALL=C sed "s|$path_standin_pattern|$path_replacement|g" old_index >.git/index
+  git commit -m 'Initial commit'
+)
+
+make_repo traverse_dotdot_trees '../outside' -x '.' '@' \
+  <<<'A file outside the working tree, somehow.'
+
+make_repo traverse_dotgit_trees '.git/hooks/pre-commit' +x '.' '@' <<'EOF'
+#!/bin/sh
+printf 'Vulnerable!\n'
+date >vulnerable
+EOF
+
+make_repo traverse_dotgit_stream '.git::$INDEX_ALLOCATION/hooks/pre-commit' +x ':' ',' <<'EOF'
+#!/bin/sh
+printf 'Vulnerable!\n'
+date >vulnerable
+EOF